If you haven't, try going into your video settings on Zoom and selecting "Enable HD". This solution has fixed the aspect ratio problem you're describing for my company's customers in almost every case.
No, Linux. I think I have found out how to use gphoto2 and a direct usb link instead of the capture card, which may fix it, but that'll have to run on my main computer, not my slow laptop.
I'd love to be able to tell the v4l driver for the usb capture card to just not offer the 640*480 resolution. That would fix it in a better way. Anyone have any ideas how to do that?
That's ... exactly what I just said. However, the transcoding burden would be a little too much for the Atom in my laptop, so I'd have to do it on my main computer instead.
Regarding the capture card. The aspect ratio mess-up is entirely its fault.