Indeterminate. Galactus COULD in theory start consuming Agamotto's realm as he did (or threatened to do) to mephisto- that would boost his own powers while weakening/killing Agamotto: even if he is "supreme" in his own realm (as mephisto is.)
moot point though. since he didn't try it, we don't know if that tactic works or not.
Galactus. the fight would have turned serious once Galactus realised that the wierd looking bug creature was quite powerful.
Galactus didn't really take Agamatto seriously in that fight.
I don't think there is any power within the universe that Galactus could not consume (and gain power from)... also, there's that handy little part of him that goes by the acronym UN... heard that's a pretty powerful little toy.