Pallas What fingers should I use for the bar 30 upper mordent (CBCB)? I think it's 4343 because that's what seems to fit the recommended fingering. I want to make sure, because in this piece, the lower mordent on the C uses 321 (switching fingers to move the hand rightward for ease of fingering what comes next). I just don't feel 100% secure that my answer is correct and validation would help.
Just a nitpick - that's a trill not a mordent. A mordent is 3 notes where you go to the adjacent note and come back. A trill can have more alternations.
Regarding your question, I would advise against playing ornaments with any other fingers than 1, 2, or 3 until you're really good with those fingers. 4343 is hard even for me. I use it only when I have no other choice. Since your teacher wants you to play all the quarter notes detached just lift your hand and play whatever fingers are easiest - probably 2121 or 3131. Both the adjacent 1st and 3rd beats are detached so you have the freedom to reposition your hand.
Pallas So I need to pay close attention to the seesaw, where there's a RH 8th note striking when a half note is lifting in the LH, yes?
I know coordination is hard in the beginning and initially you have to think about specific sequences of movements (play that finger, lift that one, etc.) but one of the best advices my teacher gave me was to think in terms of beats and fit the other notes to that. Mentally you have to think ONE-two-three and feel the pulse of the smaller notes at the same time to know when they are supposed to be played.