Clay won't get rid of swirl marks, it will just get rid of contaminants that are stuck to the paintwork. You'll need a decent polish/pad combination on something like a Kestrel DAS6.
As for the rear quarter, machine polishing might help or it might need wet sanding or even spraying again.
My tailgate had paint in December and over the winter got progressively worse. I used Menzerna FG400 on a white Menzerna pad and after a few passes with the da got it looking like this:

I spent a bit longer refining it with some other pads/compounds and it ended up like this:
