The two unchecked letters in that corner are Ls.
The preamble would be clearer if it read, 'Solvers should identify another of the characters (15, four words) by highlighting eight cells' because they're repeated words that form a circle of 8 cells.
Hi all, still struggling with 33d. And 35a. - grid full apart from that. I've googled the corrected letters in many ways and the first 4 I have are FOTS which I can explain, e.g. roo(k)f with pigeons = loft. Is it French? Is 'Gousset' part of the corrections? Any hints greatly appreciated! Thank you