Evening !
Your points:
1. ZONE INOP on a zone temperature is not a big issue. The temp controller switch would just be placed off, and that zone would uncontrolled as far as temperature is concerned. If all 3 zones were INOP, a constant 24c would be produced for all zones....(same as turning the trim air switch off !)
2. STBY pack control is used with a fault with the auto-temperature controller. PACK INOP is the EICAS message...which could be either....a)fault in auto temp controller OR Excessive Pack OUTLET temperature. STBY N,C,W is used to regulate a crude mean temperature of air to the cabin...namely 75,65 and 85 degrees respectively.
3. No..you don't turn trim air off, and YES it works in STBY (assuming you still have trim air switch turned on), as trim air warms the cabin to the temp set on the stby switch....65,75,or 85 degrees. The packs satisfy the compartment with the coolest demand, and trim air warms the other two zones.
Hope this answers your q's !
Cheers
Mon