I have heard of and experienced a problem that might be the one you're having. Do you use Discord? If so, it may be a problem with discord overlay conflicting with the game, most people who have this issue can fix it by going to settings, and finding Overlay in the app settings section on the left side, then just turning off "Enable in-game overlay." If that's not it or you don't use discord, I'm not sure what else it could be. Hope you can fix it!

As for you problem with the limited number of map models, is there a chance it could be circumvented by packaging several of them together? Like for example, several of the buildings have awnings attached to them, or generators next to them. Perhaps you could edit the buildings and add in the models for the objects around them in softimage before you export them? I'm a complete and utter noob, so I doubt this would actually work, its just a thought I had. I hope you can figure out a way around the model limit, this map has so much potential