1. No. Well, not officially. One is assigned a particular slot. Newest is a loader, then driver, then gunner, and then TC. As you hang around, the NCOs will let you 'try out' the next level up to get some experience before you are officially put there. However, here in Iraq, we're sometimes rotating gunner driver and loader if they've already got the experience, simply to spread the joy of the heat around.
2. Yes. God-awful hot in the summer, god-awful cold in winter. Noisy if you take off your helmet. (Tankers frequently have hearing loss from the turbine)
3. Yes. Indeed, you can even find (usually) your name on the smoke grenade boxes, like a fighter plane. However, some swapping in the short term is not unheard of to fill in gaps for leave, sick call, and so on.
NTM