My opinion, for all it's worth (not much, but there you go):
Stand Up
Standing up from a prone position requires a move action and provokes attacks of opportunity.
Back to the original question first:
If you are casting a spell that requires a standard action and provokes AoOs, I'd allow you to instead use a full-round action to cast that spell, no problem, but it would still provoke. By the same token, if I were to allow a character to stand up as a full-round action (expressed if you like as prone-to-kneeling then kneeling-to-standing), it would still provoke an AoO. It requires one type of action, you wanted to spend more time doing it, fine by me, but you didn't use any rules that explicity removed the bit about provoking.
Because even if you wanted to go from prone to sitting, sitting to kneeling, kneeling to squatting, then squatting to standing, the net result is that you stood up from a prone position, which "requires a move action and provokes attacks of opportunities."
If you wanted to drive from Orlando to Miami on the Florida Turnpike, you have to pay the toll. If you wanted to instead drive from Orlando to West Palm Beach and then from West Palm Beach to Miami, you'd still have to pay the toll, because you've still been driving on the Florida Turnpike. You don't escape the toll by dividing up the journey into smaller steps. If anything, you risk more tolls if you get off the toll road then get back on for the next step of the journey.
Now, my interpretation of the Prone Attack feat is that the bit about standing from prone provokes an AoO because the feat changes what type of action, but it doesn't explicitly remove the bit about provoking.
Unfortunately, there is some inconsistency in the rules about how feats are written in regard to changing an action type to a free action. Some explicitly state that the free action doesn't provoke, some expelicitly state that the free action still provokes, Prone Attack does neither. In general, unless it's otherwise specified, I'd look back at the original type of action and whether or not it provokes. In this case, standing from prone provokes, so unless the feat tells me otherwise it still provokes even if it's now a free action.
I don't hold that it's "clear to me" or "explicitly stated in the rules," but it's the way I interpret it. It would be nice if the feat actually stated either way, in order to maintain some consistency across feats that turn some action into a free action, but that may be too much to hope for.