Hi Rats
"Further, why do some engage A/P at 400 on 2 engines, when life is normal, but wait until flaps up on SE when life is non-normal? "
One answer is that when SE if you engage the a/p too early you might get a descent just when you don't need it, the a/p will attempt to maintain a speed rather than the altitude. Once in altitude hold at 'up speed' ( ie flaps up) the a/p is engaged. I guess the a/p isn't as sophisticated as we'd like it to be.