Can I be promoted to SGT in an MOS I'm not slotted for?

AR 600-8-19, 2-2e: Soldiers will be promoted only in their PMOS (Primary MOS) or CPMOS (Career Progression MOS). As an exception, both USAR and ARNGUS Soldiers who must be retrained in a new MOS as a result of unit reorganization, relocation, or inactivation may be considered eligible for promotion. This exception is based on qualification in the former PMOS and provided the Soldier enrolls within 12 months following assignment in the duty position and satisfactorily participates in an approved formal course of instruction leading to the award of the MOS within 24 months of assignment to the duty position. A Soldier will be reduced if not qualified in new MOS within 24 months of assignment to the duty position.

So yeah, I wouldn't worry about being in the 42A slot - the unit can slot people wherever they want and slotting doesn't affect semi-centralized promotions. The important thing is that you are fully qualified in the 12B MOS and that it is listed as your Primary MOS on your records. It sounds like you said you were currently reclassing? If so, you will be eligible as of the 8th of the month following you becoming fully MOS qualified, just be sure to request that your new MOS become your PMOS either through your school or personnel office.

Nope, not if you don't hold the 42A PMOS, SMOS, or AMOS.
If you do, then yes.
Technically, you are supposed to board with separate packets for each MOS you wish to be considered in.

Yes, but you have a moral obligation to those you will be in charge of, to spend a significant part (50% or greater) learning the new MOS. It's hard to lead when you don't know where your going.