8che.
Yes and no.
There are places in the world that do not have external Air con supplies.
Double Oscar,
It is a standard procedure to supply a 3rd GPU if APU is inop. Yes it means you have to have a qualified mech/tech/eng on site but if it works don't fix it. Closing the aft ext power panel with right eng running is no big deal with appropriately trained personnel.
Using just the forward ext power means only one starter motor engages during engine start. The 3rd aft GPU means both starter motors will work.
You CANNOT start the engines with just one GPU attached. 90KvA is not enough.