Seth can easily AA gief with dp, fst.mp, or fst.mk, no idea why he would ever choose to use stretchy arms. If he has enough time to react and try to fst.hp you, he can just throw a safe boom instead. To answer your question OP, yes it's probably possible if you empty jump into exgh range and Seth attempts to AA you with fst.hp, but if that situation comes up consistently you're playing a terrible seth to begin with.
Nope, Seth can't AA Gief if he's outside maximum fst.mp or dp range (though late forward jumping a lp.boom from full screen gives Seth enough time to dash forward DP). Seth can't use U1 while there's a boom on the screen. You can probably stuff fst.hp with a j.mk actually.
It's all frames and hitboxes. I assume Gief would be hit while he is landing from a jump but if Seth is too late then you will be able to EX GH before you are hit and blow through it. Not that I'd recommend doing that.