Even with the myriad pointers here I’m still none the wiser with 4d, 6d and 12ac.
6d there’s a Greek letter that fits but has zero parsing. 12ac I learn from this thread isn’t mathematical as the mention of 14ac in the clue might allude to. Second word looks like it can only be one of two.
4d, I can see something specific about the order of the letters in 23d but from the currently 222 possible words on the “solver” on this website, I cannot find a word which would describe that, and also be a diagnostic instrument.
24d I can only see the jocular phrase referring to the use of one’s grey matter, completely unfamiliar with typesetting.
28ac a contender for COD (so far!). 24ac also raised a smile.
I’ll go and jump in the shower. I don’t know what it is about application of water to head, but it almost always unlocks something.