I think the P51 fly-over occurs once the Germans cap all flags up to the Alamo, whether the Alamo itself needs to be capped also, I am not too sure.
I have had times when my team tried to get over the bridge but couldn't, holding the Americans at bay from recapping the other side of it, and yet I still saw P51s eventually
The spawn locations have to do with the new FH2 team-spawn i.e a flag has 2 sets of spawns, one for each side. So when the Germans spawn at one base, they spawn more towards their side of it, and not directly on the flag. In a map like Ramelle, where flags are so close together, that's practically the next flag