I'll be using The Ball's (heh. funny. ) theory post for alot of lingo and stuff as to keep it simple, but this is more or less my idea on whats happening in engarde and it'll be sorted in areas you'd reasonably reach first.

The Start.
When you load in you're either in the motel rooms, the garage of the gas station, or just in the road. This is my opinion could mean three different things.
Motel Rooms: You're a civie who's been spending some time in the riverside town, you either slept through the event or you refused to leave your domicile for fear of your own life up until you mustered the courage to leave.
The Garage/Gas Station: Similar to the civie spawn, you're either the owner/stationed worker of the gas station or someone who came to get their car fixed and found the town in this state. I'd lean more towards the car breaking down rather than working at the gas station.
In The Road: Using the description as a inspiration, you're either a civilian who stumbled upon the town, or you're apart of the mentioned " Reponse Team. " But i'd lean more towards being a random person as its implied the team was...KIA.

As you look around the starting area you notice not only that each room except for two are vacant, and the gas station still seems stationed yet vacant of course. Theres a room boarded up next to the washing machines in the motel area thats locked, but theres a room in the main rooms where a woman is in the closet. As you open the door and investigate you notice the dead body in the bathroom. followed by gunshots and someone getting attacked. we dont see what attacks but now the boarded room is open and theres a dead survivor inside. He looks already far into Pallor mortis, which is what happens when the blood stops circulating and goes away from the skin.
When you look through the back door you see the outline of something. using a closer view its a mannequin.
Remember the mannequins.

Exiting the room, you can either head down the road, or past the gas station. We'll head past the gas station for this.

The Forest.
Venturing past the gas station you have the chance to hear an..animal or something off in the deeper forest. Theres nothing particularly interesting in the forest but as you get past you see a shed? no a bathroom and theres something inside letting out a loud noise that doesnt sound human, or animalistic, but inbetween? Sadly the bathroom shed cross thing is locked so we cant see what it is.
The Bridge.
Heading back to the main road and we past over a bridge and in the water is an unnatural amount of cars, It doesnt seem like they're accidents since obviously the bridge is intact. We'll save this for later.

The Town.
The first thing we notice is that on the right, there is a sign that mentions military presence. Aswell as there's a house that's locked, boarded and has danger on it. We cant enter it, even with hacks but there's an obvious blood trail that leads into the cornfield. Despite my best ability i could not find anything interesting in the field so i'd assume the trail lead from the field to the house and not vice versa. On the left is a house that is also boarded up, but behind the house we can find a window thats breakable, Upon entering the house we can search it to find weaponry, boards and a US ARMY Survival Guide, Clearly this person was ready, but they're not here. Searching upstairs we can find a locked passage that we cant enter, aswell as average stuff like clothes beds and stuff however in the closet is another **mannequin.**.

Moving past the house we can find another Danger House. I'll keep the rest of the houses short as to save on reading time.

Inside we find a blood trail that goes upstairs into a locked room that we can enter from the bathroom.

In the second right house its fairly tame, but in the bedroom we find a man in black painting. a Memory Marker as balls calls it. Remember these for later.

The Train Yard(?)
Focusing on the Train Yard area first, we find that the gate is closed but is easily opened by the building next to it. Heading inside we can notice its fairly tame, just trains cars and stuff reasonably in a yard. Heading north and upwards theres nothing interesting to the right, so we'll head left. Ignoring how the light break made me change my underpants, we find a campfire next to a hole between two buildings.

We'll ignore the hole for now and investigate the buildings. Left First.

Left Building: We'll Head back down and turn left, coming across a gate door and heading inside. Climbing the ladder we find a survivors place without and survivor, but thats all.

Right Building: Heading back past the campfire we can find the entrance to a facility and we enter. As we search we find a Functioning buggy. its not particularly relevant but good to have. Theres a security-esc door that upon entering, we find another Memory Marker and a empty chair.

The Edges.
Upon primarily heading left, you find a sort of logging factility? upon a closer look you find multiple dead bodies. too many to be reasonable and they're all deep into pallor. This could be a feeding ground for whatevers in the bathroom. But behind the main building is a large steel door that seemingly connects to something.

The Campfire Hole.
Heading down the hole, we find yet Another Memory Marker, and continuing i'd assume this is a sewage drain repurposed into a hideout? heading left we notice immediately theres blood, that leads to multiple locked rooms. Heading left we find its much more broken, like a raid happened here. Theres signs that this was a rebel post since we find multiple symbols for the rebels, and places with ammo, living quarters, and gas. Heading down the hallway we find. Ken? a mannequin torso laying in the middle of a hallway. Ironically nothing happens if you shoot him so we'll keep running. Eventually you get to a open room with a ladder that leads further underground. We'll end it here since every tunnel we've found so far connects to a place i want to talk about.

The Engarde National Laboratory: Immediately we notice the military fortifications, cars and overall lack of people. Heading inside we find a quarantine zone sign on the wall, obviously not good. We notice theres stairs and multiple doors, clicking a button in the receptionists desk does something so we'll find that out. Upstairs we find a shotgun and some shells. Good for us. Downstairs we notice theres a elevator, so i'd assume thats what the button opens. Heading down theres a short ride to a locker room and parking area with some military vehicles in it. Upon heading left we find a long hallway that leads to another elevator.

The Laboratory: Upon reaching the elevator we see multiple ways to go, the stairs, the elevator, and to the left. We'll search left. The first door on the right leads to a armory so stock up i'd guess. The hallway on the left finds a security(?) room that has a button, im not sure what it does as the elevator has always worked for me but we'll press it. Heading all the way down we see a ladder and a button. Upon pressing the button a hatch closes down and it leads into the house with the memory marker..Interesting. Returning to the elevator i encountered an error where the metal chain doors close, but the elevator remains and the heavy security door doesnt close. The doors did eventually open but we'll move on.

As the elevator stops we find a underground train area with a big metal door and a security checkpoint similar to black mesa in my opinion. moving down we find a HEV suit and computers. Interesting. We head up the stairs and move into the security door, opening the heavy doors because our eyes are apparently just adaptable. We find various scientific meeting rooms, offices etc, untill we eventually come across a room overlooking a server room. If we look across the room into the opposite meeting room...we see a familiar figure. Breaking the window out and climbing to the room to our left we can enter a hallway that leads to the room. unfortunately, its too late and theres only a memory marker left. After a while of searching, atleast for me. i managed to find a female civillian NPC who started following me. It caught me off guard at first but i kept her around. As we search further i eventually came across a room with multiple HEV suits, a room with plants in a cell, and a deep dark water pit. I walked down the hall past the HEV suits door and she vanished. Next to the HEV suits we can find multiple surgery esc rooms with even a combine on a table. Heading into a particularly bloody room with cages, we can press a button to call an elevator. So obviously we'd head down. Upon flipping a button, we find a dead zombie corpse, followed by various other corpses and...another mannequin + memory marker. Jeez.


That's all ill talk about the map itself, Onto the theory itself.

Obviously the entirety of this town is based around the laboratory itself, very few if not no civilians lived in this town. Judging by the fact the laboratory has multiple secret entrances, various tunnels and proof of experimentation that all are hidden beneath a town that despite my best chances, had no actual stores or anything that would prove people lived there. Even the laboratory itself was a fake for the underground complex it covers. This place seems more like a black-mesa that instead of being hidden in the middle of nowhere, was hidden below a town. As for the cars in the river, i'd assume the motel and gas station were the only things actually intended for civies. People drive in, rent a room and fix their car. As they're spending the time there they are kidnapped and taken into the underground facility to be tested on, which would explain the cages in the surgery room with people in them. I'd assume there we're also headcrabs most likely sourced from black-mesa as there are zombie corpses occasionally. My best guest is that the mix of plants, headcrabs(atleast the shadows i've found of em), and civilian test subjects tells me ( and the confirmation in the balls theory post. ) DNA splicing / trying to get the parastic mind control of the headcrabs to work on plants and similar organics as to be used as a form of..easier mind control? My best guess is that due to their attempts at genetically modifying stuff from other dimensions with stuff from theirs resulted in a batch of Gonomes or similar zombie types that also have elements from plants and such. Honestly if its not directly half life i'd absolutely guess they made a bad batch™ of those fallout spore carriers and stuff from vault 22.

The MOIs ( marks of interest. )
Mannequins.
Honestly, i think the mannequins are a form of appearance for the created monsters. They appear everywhere you might not expect but it makes sense for a monster to be. Theres a mannequin in the survivors back room after he dies, a mannequin in a house where there should be a survivor since theres guns and ammo, but noone to be found, and a mannequin wheres theres plenty of bodies thus food and blood to drink. The final mannequin i've found was in the rebels base, where a massacre happened. I'd assume this mannequin was a monster that was one of if not the only monster to have been killed, since it has no limbs other than a head and torso.

Memory Markers.
My opinion is, someone or something leaves behind these markers for places where something important happened or is to be remembered. Theres a marker in the second house that had a hatch in it, a marker in the room over the campfire, a marker outside of the rebels base in the trainyard, a memory marker *in* the rebels base, a marker with the bodies underground, and a memory marker where you see the familiar figure.
Each of these can have some form of meaning.
The Second House.
This memory marker tells us that the town isnt entirely as it seems, showing us that its only a ploy for the underground base.
The Rebels Campfire.
This marker shows us that even while everything was going to hell, people still managed to stay happy.
The Rebels Base.
This Marker marks a base where the aformentioned rebels took down a monster, and died afterwards.
Another possible meaning is that this is where a strike team raided and killed both the rebels and the monster but the lack of bodies says otherwise to me.
The Marker in the Dead Room.
The Marker in the dead room is most likely for remembering that, this facility took innocent people, and used them for experiments that go against the will of creation itself.
and the Marker where G-Man is.
This one speaks for itself as G-Man is a prominent figure in mostly every base. He isnt particularly seen by anyone other than gordon as far as i know so he's clearly here to check on the other variables while gordon is off in new mexico or something.

As for the misc things, the colorful sky and the date mostly lines up with the 7 Hour War. as black mesas event happened on May 16th, 200? and the 7 hour war happened later. which could line up with the date of the map as the date on the map is 9/5/200? there isnt a confirmed time for the incident but i'd assume for this maps lore its that day.