Based on cooperman's suggestion, I ended up fitting a 2N5322 (BSS17) in place of the green 16506 part at location 624R, and a 2N5320 in place of the grey 16505 part at location 625R (after checking the other passives and actives first). They seem to work great - the right channel is back, both channels are nice and balanced, and very clear 🙂 Thank you all for your help!
https://www.youtube.com/watch?v=yX2XpNJ-yhA