"Certainly a diagonal "express shaft" would seem to run contrary to the idea of the lights being shaft wall lights visible through lift cab "windows", as the cab would have to rotate from the shaft angle to the deck angle whenever our heroes enter or exit, yet the lights do not reveal any rotation."
No, not necessarily. The lights in a diagonal shaft (in which the turbolift cabin is of course keeping a vertical position!) are parallel to the deck level. If you take the standard situation (turbolift cabin interior, turbolift door the right) you can step out in the direction of the bridge / the ship is moving towards in the saucer section and the engineering hull. Also if our protagonists exit the turbolift towards the starboard side of the ship (end of circular corridor) the rotation doesn't have to reveal a red turbolift door where the turboshaft lights should be.
While at first sight the idea of the cabin windows just being flat screen panels appears interesting, I think they would put it to better use by displaying a ship's schematic that tells the user where he or she is (compare TMP). Which reminds me that obviously they did change
the turbolift structure in TMP, otherwise Admiral Kirk wouldn't have asked where to find a certain turbolift after he got lost.
And regarding the size of the turboshafts (no, I'm not thinking of the Earth Spacedock turboshaft from ST V) I'd like to remind that in "Where No Man Has Gone Before" we saw a definitely rectangular turbolift-box that seemed big enough to also have enough space for a stretcher.
So we might be looking at various turbolift cabins, too - small or big - especially since we have never seen the inside of a TOS turboshaft.