HW7@20, I simply do not understand what you mean, C is (are) added to a letter adjacent to the structure - so what do you mean by centres of the relevant cells?
28a A letter in the clue moves three words forward. Unusual anagrind for the last three letters.
27d It’s listed as a variant under the main spelling in Chambers. Change the first word, reversal then two-faced means duplicate the first letter of to tout
I think I’ve finally gotten all the bits of the endgame, but I have one parsing element that’s still eluding me—my relevant letter in 26ac seems wrong, but I can’t find a way to work the apparently necessary letter into the clue modification. Anyone have any tips on parsing this one?
Oof, ignore my post. I just tried entering an unfamiliar word into chambers simply to make the letter work and it turned out to be relevant. I had previously created a different example of the same broad category of word for a basically identical definition.