Hello Ivan! Max is still in development so all I can offer is an educated guess based on where its heading so far. Everything I am about to say might change.
Quote: "1. Will it be possible to have a co-op kind of multiplayer? If I remember correctly in all the previous GG versions it was just like a deathmatch."
Earlier on in development it was stated that the multiplayer will be improved in terms of stability and quality of life but it was also implied that it will still be deathmatch. However, given that MP has not been developed yet, its possible that we might have more freedom when it comes to custom scripting during MP. If there will be any pre-made stuff to help you make a co-op game remains to be seen, however MP was mostly an afterthought in FPSC and GG and thus never really worked well. If this will change with max remains to be seen.
Quote: "2. Will it be possible to have both multiplayer and single player in one executable? If remember correctly you had to choose if your game was mutiplayer or single player in the past."
No info on that one yet.
Quote: "3. Will it have an easy ragdoll option? Maybe this is more like a feature request and maybe it already exists and I haven't heard about it but it would be nice to tick a box and have characters flop when they die."
That is already the case for GG and FPSC so most likely.
Quote: "Will I be able to get my NPC's to interact with my other models?"
Character navigation in GG classic is far better than what it was a couple years back and they can navigate models relatively well now but not up to any contemporary standard. GG max will have navmesh so characters can scale stairs and ramps quite well. As far as programming in contextual NPC interactions (character reaches a bench, sits down etc.) ... no info yet.
Quote: "5. Will animations import even if they don't utilise a skeleton for the animations?"
I think that this would be very unlikely but we'll see.
If I where you, I'd check back in a few months and see how things progress as many of these things are just not established yet.
-Wolf