To answer question #2 and 3, trim air is often turned off on the ground to prevent the PACK(s) from icing up. In hot and humid environments, and especially when performing max cooling on the ground (PACKs in HIGH) PACKs can freeze. Normally the L PACK to be exact. This is because the aft portion of the aircraft is most difficult to cool, so the zone temp selector should be the coldest temperature selected-- say for example 65 F. While the flight deck is demanding 70 F. With lackadaisical use of the fwd entry door and the flight deck door, it's going to be nearly impossible to reach 70 F in the flight deck (or CONT CAB). So that L PACK is going to work really really hard to cool down the flight deck. And here's the kicker-- that trim air switch that modulates bleed air to satisfy temperature demand, doesn't always work perfectly. So sometimes, since the CONT CAB (in this case) isn't the coldest zone temp control, the trim air valve for the CONT CAB has been known to erroneously open and close; effectively not modulating itself correctly, and therefore slows the cooling for the flight deck EVEN MORE, requiring MORE juice out of the PACK.
This was the drawn out, mostly laymen's language way of describing why we sometimes turn the trim air switch off. But the easy explanation is, our most current max cooling on the ground procedure tells us to do so. Granted, I'm a Navy guy and not governed by whatever checklists United pilots fly.
Lastly, it doesn't matter what position the switch is in for engine starts. Normally I'll leave it off for starts, and when I configure the bleed air control panel during my before taxi "flow", I'll turn the switch back on.
Hope this helps
