Question 4: there is no cooperative mode
Players sometimes like to form private teams to play games with each other, which requires cooperation and a little bit of story pushing
Solution Suggestions:
Cooperative arena mode, players can cooperate with each other, no weapons, enter the arena, before starting to pick up their own weapons. Once in the arena, players need to cooperate to survive and resist waves of attacks. Each time they survived, they were able to resurrect their dead teammates and get cheers from the crowd.
Arena maps are very much available in single player and can be ported directly to multiplayer