1. I'm afraid that's not possible, because team weapons are fundamentally different to small arms in how they work. Team weapons are separate entities with squad members being attached to the gun, while small arms are more or less just animation states shown on the entities.
You could make the Maxim behave more like a slot item by giving it an instant setup and better movement stats. Alternatively, you could give its stats to a slot item like the DP-28.
2. Each weapon has, under "behaviour", a flag named "attack_ground_type". This needs to be set to "anywhere" for barrages to work while garrisoned.
Do I need to add texts into the abps files?
What do you mean by that?
3. I have tried that as well, a while ago. It seems that the marker for the crew position is set wrongly, so I don't believe there is anything we can do about it, sadly.
I'm sorry that I couldn't provide any more help, but the mod tools are quite limited right now. Feel free to ask any more questions you might have, though!