@Lo1snake I've never really gotten on with the crossbow launcher, I'm a limpet fan myself, but the AT one is alright to finish off a tank (I wouldn't try to solo one lol). With the frag version I think you're best chucking them down corridors or at clusters on enemies.
I'm not so sure about not being able to repair. Any class with the repair tool should be able to repair from inside or outside if memory serves. Hopefully a PC player can help you with this one. The only other thing I can think would be the bug where you spawn as cavalry inside the tank, but I doubt you'd get that every time.
Learning spawn points is mostly just experience. I personally hate spawn camping so don't do it myself unless I'm passing by and an enemy coincidentally spawns in, but that's mostly self-preservation.