Considering I see people commenting a lot about the mod playing as Balin I had to give my take on what and how while playing the lore wise doomed expedition. It is not impossible, just very improbable. Problem with current version is that same starting event procs twice instead of once. So surviving the 15% chance twice is harder. Generally Balin starts as a diplomat, big chances for that one alliance perk, usually Aragorn or Elrond for players as it sticks for as long as Balin is alive (even if he doesn't have the perk you can reset perks to get it). With their troops and your starting dwarves + 1 pack you take extrta from 200 gold you get at start should be enough to defend yourself. I did it without alliances so you can do it with them. Without you just hold the line as they come, you should be able to defeat them in your territory. Personally I loved that part, felt like I am changing the history by us holding the line in that final chamber but we push the Orcs back, feels rewarding.
Problem you'll have after that is sucession. As Balin is badly wounded his fertility is a problem, he is quite old already and he is asexual so his chances for offspring are quite low. Problem of the game right now is he is not only badly wounded but mauled so his fertility is -75%. And it takes forever to get rid of it. Took me 40 years to get rid of it myself. Only after that with constant seduction of my lustful wife did I get an offspring. In my 3 tries as Balin non of my kids got to 20 years before Balin's death from natural causes that tells you enough how problematic offspring is for Balin.
As for strugle, there are ways to fight through it. Take the orc bases closest to you, spread to mithril mining areas, one is under orcs, one is next to you. Get mithril base up and focus on growth. Honestly, if you don't build armies like crazy, you can relatively easily get enough gold to grab new settlements and then abandon them to get Balrog to a state of dormant. while dormant build up bases, get gold to make him go dormant as fast as possible when he reaches wandering status. DO not let him get active, let alone Armaggedon. If you get him active (honestly imo only possible to happen at start) you can try to get him active on purpose at start if you quickly grab neigbour orc bases with help of Aragorn or Elrond, build them up, take two settlements but don't abandon them yet. Get him active but save gold for 2-3 settlements. When he gets active take out your settlements and all gold goes to get new ones that you will abandon to get him to wondering as fast as possible, then just play it so you abandon settlements to get him dormant, all the while building your base slowly up. Ending the struggle is a different set of issues. There is max 50% chance to kill Balrog, AND there is a 50% chance if you kill him that you will die anyway. So do it only when you are feeling really strong in Moria aka Balin will not be doing that. It will be either his son or grandson.
One more thing, when you are playing Balin you are playing Moria, it a small world of it's own inside the LOTR map. DO not think or even look what is happening everywhere else in world. It is useless to you. Struggle of Moria almost on any way cannot make a difference for the grand scheme of things. It will take around a 60-100 years to solve Moria struggle or 200-300 depending on many actions, only after that much time will events of the rest of the map matter at all for Moria. So Khazad-Dum is not played by players who think that dwarves of Moria will kill Sauron aka stop him from assaulting Gondor. That is not realistic. Their job is to take out Balrog and make a strong mouintain kingdom.