Just my opinion:
1. I'd think at least 6 at a time on a 42. I've done two on the main grate of my 23 and pretty sure my 32 could handle at least 4. To size it up, just lay them all out in their cryovac on the grates until it's as full as you'd be willing to go.
2. Matter of personal preference and what you're comfortable with. My advice would be to stay with what you know and not mess around. Personally, I like 275 for brisket, but that's just me.
3. Same temp will give you the same result. At 275 I think you'd be looking at around 10'ish hours.
4. I wrap in pink butcher paper just after the stall and this does shorten cooking time. My 10'ish hours estimate above is based on wrapping. But, again, I'd stay with what you know. Or you could try doing just one at 275 with butcher paper wrap and see how it goes.