1) Well ofc it`s possible to add new units in game send massage to grimeleven or bladeking77 or any other moder who imported some new unit in his/her mod. What i`m sure is that you must add 4 whole files for unit, i`ll example champion from adventure mod :
champion.atom (unit stats and abilities) , champion.png (unit picture) , army_champion.atom (unit model, sound . . . technical things), champion.bma (i have no idea what`s in it) , champion_diff (also no idea what`s it for). But there other files that make unit work in game colision_champion.cms , addon_unit_features.lua (how certain unit special ability works) , eng_units.lng (lists all units and their basic information that are shown when you right click on them in game) , special_attacks.lua (also for unit special abilities) , special_params.txt (lists units that are affected by some item or skill, like inquisitors blade lowering paladin lds etc) , unit_features.lua (explains unit talents and pasive bonuses, like orc adrenaline , or rune mage runic armor, etc) , unit_morale.lua (obviously for unit/race moral bonuses/reductions). So for 1 new units you need 4 new files for that unit, and to change 7 other files to include that unit in future games , and it`s not as simple as copy/paste couple things in locs folder (and why there in first place ???), all unit files start in atom folder, and then scripts folder and so on ...
2) . . . no more ideas
3) i my self made 99 skill lvl`s, but limited pet to 70 so to make some choices what skill to develop in what game , and not getting them all on max I`m glad i helped you at least with this..