Welcome back!
- They can (like any OT crew), but it usually means that they, and especially their passengers, can't fire effectively (if at all). Passengers and crew BU together.
- Same as a regular AFV (crew nationality's best unbroken Elite Infantry). (edit: thanks Klas! cut-and-paste error)
- Again, same as any AFV...see D5.311:
...The Inherent crew (as well as each Passenger) of a vehicle receiving fire (including Fire Lane Residual FP) [EXC: Sniper attack] through an unarmored Target Facing/Aspect (C3.9; including any OT AFV receiving either Air Bursts or fire from a higher elevation whose elevation advantage is > the range [5.31] — and, vs mines, the PRC of any AFV with a hull AF of 0 [B28.43]) is Vulnerable even if BU, and receives no (or a reduced; 5.31) CE DRM. Consequently the crew is not susceptible to Stun/Recall from such an attack; it is instead subject to PTC/MC/K/KIA results. If it breaks it must rout from the vehicle using normal Infantry rout procedures [EXC: it expends all its initial-RtPh MF to be placed beneath the vehicle]. The vehicle automatically Stops (no Stop MP is spent) when its crew breaks. If the crew routs or is eliminated, the vehicle is marked with an Abandoned counter.
4. The standard vehicular crew counter (the lesser, 1FP, crew in each nationality's counter set).
I think all of your questions are answered fully in Section D5. In sum, Halftrack crews are no different than regular AFV crews. Going BU, however, is a useful way to transport passengers into danger (provided you don't want them firing their weapons along the way).