Galactus can manipulate matter itself, matter in general as it does not specify what matter he can manipulate, and he can do pretty much whatever he wants with the Power Cosmic via matter manipulation... like in the link I provided.
Giant cake - Providing he's outside Buu's head, Buu can't get distracted by what he can't see
We've covered this, buu can destroy reality. Getting sent back in time would just annoy him.
Actually, all your points are useless. Treach, the person who is defending Galactus has already revealed that Galactus is eons old. So is Galactus going to send Buu to a time when Galactus is, arguably, weaker than he is now ?
Matter, without any specification, generally concerns all matter, no more, no less.
He can restructure molecules or convert the matter of molecules entirely into anything else... Mhhhh, Pie for Galactus .