Without your prompt I don't think I would ever have found the elusive P.
An excellent puzzle, with lots of good clues and a fun endgame. There's a few I haven't fully parsed but I'll leave it for now and look again with fresh eyes tomorrow.
son1ofrover1 - I'm not happy with my 'solution' because there's no dialect indicator in the clue and the abbreviation is capital 'A', but the 2nd entry in Chambers gives - 'A a dialect form of I'.
I think the preamble should have said "correct letters", not corrected ones. In one clue, for example, an F should really be a T, so the F is corrected, not the T. But it's the T we need.