Update! Yes, people on Steam will be able to play co-op with those on GOG. You can't use the "Invite Friend" functionality across platforms (as they don't share friendlists), but you can find each other's rooms in the browser or join via the room's join code (a unique 2-3 word string that is unique for each hosted room).
There can be only one AIM merc in the campaign. We went back and forth a couple of times on this, but ultimately I didn't want the singleplayer game to be differently balanced from the co-op one. IMP mercs are very powerful in that you can optimize them to fit what you need in terms of stats and perks, and their contracts don't expire. For those who want to play differently, modders will be able to figure out how to enable multiple IMPs.
Thanks for the kind words! Ultimately this is the best way to make a simulative, reactive, emergent game.
Modders might decide to tackle that, but there is no such mode coming on release. If there is community interest it might happen.
Yeah, you can modify existing models, images, stat values of items, mercs and other content in the game.
They would automatically show up in the AIM browser if you mark them as AIM mercs, otherwise you would have to mod in a specific way of obtaining them.
I can't comment on what will happen in the far future. I wouldn't rule it out, and I wouldn't bet on it either. Let's cross that bridge when we get there.