I ventured down this rabbit hole a few months ago. I got to the point where I had a semi-working solution, but it was very fragile. So in the end I abandoned the idea. If a user asks for their email address to be changed, I just do a global search and replace in the Google Sheet. Simple. Done. Maybe they lose the state of some user-specific columns. Shrug, I can live with that.
On email vs RowID, I try to use RowID to identify a user wherever I can. I think it’s a better practice.