Re: "late" Powell, I'm late to this discussion, but here's my take:
Powell in '53 (mostly live recordings) was OK, but a less nimble version of his former self.
Most of the later Verves and RCA recordings to me are painfully bad.
For the later Blue Note sessions he seems to have come up with a modified, more hard bop and less bop-virtuosic style that worked well enough, but couldn't compare to 1946-51.
From '59-'63 he seems to have returned to his classic style, and at times he sounds as brilliant as ever (e.g. Lausanne, Portrait of Thelonious, and a few others).
Blues for Bouffemont is an interesting document of Bud at the end and how erratic he was: the first session has some brilliant moments and the second session is an incoherent mess.