The intended experience for the mod is to discover the narrative threads and secret paths on your own. That said, I do understand if you want to get straight to the meat and know exactly what there is, and how to get there. So here's that list, in more or less chronological order.


How can I get Elven or Half-Orc Blademarches?
When you start as Stalwart Band, at startup, you will immediately be greeted by an event that offers you to choose which race leads the adventuring company.

Will I get anything for starting as another nation, and then forming Blademarches, such as Ibevar or the Wood Elves?
No. Wood Elves might in fact lead to some bugs, though I haven't tested it; the mod is built with Moon Elven culture in mind.
The mod is also meant to avoid the common Ibevar into Blademarches path; instead you can immediately start as Elves and will receive very similar bonuses from missions as Ibevar would. This obviously means you can stack them even higher if you do go for it, truly breaking the game - but I'd personally recommend experiencing the narrative starting as an adventurer.

What about Orcs and Goblins?
No special path either. Will in fact lead to even less content, though hopefully nothing should break.


What is the Imperial and the Royal Path and how to get them?
Upon finishing the early part of the mission tree, you will get an event with a choice about which faction of the court to support: The Imperials or the Royals.
The Imperial path represents the original tree for Blademarches, with the addition of some minor adjustments and the shared parts of the tree. This is the path if you want to conquer all of Escann, as before.
The Royal path is where most of the new content actually happens, and its meant for playing tall, limited to the core territories of the Blademarches - though there might be a possibility to break out of that eventually...

Anything special on the Imperial path?
Nope. There are some shared new missions and events, and I've reordered the old mission tree a bit, but nothing wholy new. There is one government reform for Tier 5 – Army Organization for the Imperial Path.

So what does the Royal path unlock specifically?
As said, you won't be doing much conquering. You will eventually receive:

• A fixed set of Trade Goods within your territory, including Iron, Bronze, Gold and Gems; as well as multiple Grain and Cattle provinces. This does mean that if you do happen to somehow start with three gold mines through sheer luck, this might end up being a downgrade. Development and output are boosted further by mission rewards.
  
• An army reform chain, starting you out with a transitory reform (Royal Blademarcher Army) that will provide yearly professionalism and discipline, but less Force Limit; eventually upgraded at the end. On the way there, through missions, you will also get additional bonuses, eventually merging into the powerful „An Army with a Blade“ modifier. This is very similar to the bonuses from the Ibevar tree, but adapted to Blademarcher lore (e.g. more focus on shock than fire). Eventually (only if you own the Domination[/i-dlc)] you will get a choice between two outcomes: the Bladesworn Army, focusing on using and boosting special units, and the New Model Army, focusing on Militarization. Both of these feature Power Bars and buttons to use them on.
  
• You get an modifier enabling you to send bigger stack of Condottieri, which can get upgraded in each age up to the Age of Witch Kings.
  
• An alternative Tier 8 reform
  
• As final reward for completing the tree, you will unlock the „Strength of the Kingdom“ modifier (identical to Strength of the Empire from Imperial Path/Vanilla Blademarches) and an additional Privilege for the Blade Stewards

So what is shared between both paths?
The revamped start of the tree is available to both paths – this includes the consolidation of the core Blademarcher territory, and the vanilla military bonuses.
Government reforms at Tier 3, 6 and 7 are automatically available for both paths. At tier 4, one can be unlocked by a mission if you become Ravelian and another by mission if you are Corinite. A reform for tier 8 unlocks by event for everyone. The whole religious branch of the mission tree is shared, where the Corinite path unlocks Blade Stewards Estate and Bladesworn special units at the start (only if you have the Domination DLC), as are the Racial Military missions.


What special content is there for the various races?
Missions
Humans, Half-Orcs and Elves will each get their own two-mission-long subtree, with the first mission a choice and the second depending on the choice, where you get to decide how to specialize your army. So in total, six possible outcomes.
Those choices are:
Humans: "We shall be the very best!" --> Strive to be the best a human can be, increasing common modifiers like morale and infantry power, lower mil-tech cost, and adds prestige from land battles. OR "We have much to learn from other races." --> Go on a journey to learn the military secrets of various other races, eventually merging the results into one powerful modifier.
Elves: "Ten to one? We like those odds." --> A fully Elven military, but supported by a human supply corps. Higher in quality, but smaller. Helps with some of the maluses of Elven military. OR "Elven Lives are too precious, let humans die for us." --> A Military of Humans, led by Elves. Changes to Human Military, adds some of the benefits of Elves - somewhat similar to the army reforms from the Eliznan mission tree.
Half-Orcs: "Only those of pure mixed blood shall be allowed to carry a blade!" --> Half-Orcs become a warrior elite, increasing quality, yet limiting your manpower from non-primary-culture provinces. OR "Two halves were necessary to make the whole – and so all shall serve equally." --> Orcs, Humans and Half-Orcs all serve together, leading to a larger, less disciplined army - indeed an antithesis to the normal direction the mod takes when it comes to army modifiers.
In addition, these missions will unlock one special privilege per race for the Order of Blade Stewards estate, improving the Bladesworn special units. Elves actually have two options here, depending on the path they took; for humans and Half-Orcs, they are identical.

Governments
There is one exclusive government for each of the three races at Tier 7.
Humans: "General Staff" --> Cheaper generals, cheaper Mil Tech. Also adds Monthly Militarization.
Elves: "Council of Blind Masters" --> Old Age Penalties are less likely, similar to the reform for Ibevar. Also Mil-Advisors are cheaper.
Half-Orcs: "Great Balgazor" --> Gives you a parliament (without removing Nobles), lowers fort maintenance, increases reinforcement speed.

Events
If you're not human, upon finishing the early reconstruction part of the tree, before you make the choice between the royal and Imperial paths for Blademarches, you will receive an event about descendants of the former human inhabitants of the kingdom returning, turning many of your provinces human. You have no option not to accept them here, though you would always have the option to culture convert manually. You will not gain anything special for it though.


How do I get the new estate?
The unique Estate for Blademarches is the Order of Blade Stewards. They are unlocked by the first mission in the Corinite subtree.

What about special units?
The special unit are the Bladesworn. They are unlocked alongside the Blade Stewards estate.

What are those special unlockable army reforms at the end of that mission tree?
Upon finishing the army reform mission tree, you will get a choice between two paths forward: Institute the New Model Army, a government reform unlocking a form of militarization, or
go with the Bladesworn Army, featuring a power bar built up by war victories, and increasing the amount of special units you can have.

How to trigger or avoid the hidden disaster?
The disaster, The Bladesworn Rebellion is only available if upon the conclusion of the army reform, you went with the New Model. This will add a special privilege called The Old Guard making the Bladesworn disloyal. If you then also add the privilege that forces them to Support the New Model, and then start a war (does not matter whether you are aggressor, defender, or secondary participant), the disaster will appear. If you do go with the New Model, but never trigger the disaster, you will never be able to remove The Old Guard though.

What are the possible outcomes of that disaster?
Upon the disaster triggering, you get to choose which side to support, the rebellious Bladesworn or the New Model supporters. In general, you will get the maximum choice of outcomes if your side wins and you never made concessions to the other side through an event that might pop up. If you did make concession, you will not be able to take the most radical outcomes, even if you win. If you lose, you will be forced into an outcome more favorable to the opposing side. The possible outcomes are:
Bladesworn Victory: Bladesworn get restored, enabling Bladesworn army and removing the negative Old Guard privilege. If you supported the New Model, your ruler will die. OR Transform into a Theocracy, lead by the Order of Blade Stewards. (only if you did not make concessions)
New Model Victory: Keep the Order of Blade Stewards, but replace the Old Guard privilege with one making them loyal. OR Abolish the Order of Blade Stewards, losing access to the estate. (both only if you supported the New Model) OR Create a Bladechosen Republic, where a general becomes ruler and takes up Calindal. (forced upon you if you supported the Bladesworn and lost; otherwise only available if you did not make concessions)

How can I complete the The Lady of the Lake mission?
Five years after finishing the army reform, an event will trigger – this unlocks the „Find Respite“ condition of the Lady of the Lake mission.


What path is the canon one / which one would you recommend?
Well first, the narrative in the mod diverges pretty heavily from the lore of the main Anbennar mod, especially now that it is getting expanded to lead into Victoria 3.
That said, the Royal path was originally made with that lore in mind – a path for Blademarches to stay as small, yet survive, as it eventually ends up in the Victoria 3 timeline of the mod; which implies that the original mission tree where you conquer all of Escann and the Deepwoods and form the Empite of the Blade is not canon.
In addition, the newly added change to a Republic at the end of the Bladesworn Rebellion if you support the New Model was meant to provide another plausible path into that timeline (since Blademarches ends up as a republic there).

Personally, I enjoy the Elven and Traditionalist/Bladesworn paths the most. So my suggestions would be either:
Human, Royal Path, go with New Model, trigger Bladesworn rebellion but win as New Model, go Republic, go Ravelian. This provides the maximum Militarization gain and brings you closest to the canon outcome; at the same time you actually get to escape the governing capacity restrictions of the Royal Path, so you might just conquer Escann anyways.
Elven, Royal Path, go with New Model but then change sides to Bladesworn, go Theocratic, stay Corinite. The maximum spacemarines / Prussia of Anbennar path.