I had a successful lesson using Zoom on Satuday. ... I fed a microphone into a spare channel on my m-audio 410, mixed it with the organ outputs and sent it to the microphone input of the laptop via a cable. I listened to his comments via a one-eared USB headset. Webcam was only used for video ... not audio. Doing it this way meant there was no chance of feedback.
Getting the levels right was a bit tricky, and varied with pitch ... but we found a registration that worked for him (even if it sounded odd to me!)
Whilst not ideal, it worked well enough for him to spot odd wrong/missing notes, missing articulations on repeated notes, etc. (No chance of hiding my mistakes
)
A simpler arrangement just using webcam audio and video may well work too ... I didn't try that because I mainly play on headphones at the moment and my speaker setup is pretty poor)
Good luck trying it out ...
Mike.