Client needs new domain, switch primary domain on web hosting to new domain, forward old domain (on different registrar) to new domain. Client loses access to emails from old domain. Restore settings on old domain web admin to 3d pty hosting (IP & name servers). Client still can't get emails, and old domain returns unknown host in browser.

Solved: by hosting the old domain we were able to edit the zone settings, which weren't exposed in the registrar's web admin.

Well that's just rubbish... If you (not YOU, you understand..) give a client control of this stuff, why is it only partial?!

Not really the registrar's fault, for whatever reason the guy who configured the email used zone settings on the web host (even though MX pointed at google mail servers), which I unknowingly nuked when I switched over to the new primary domain. When I added the old domain back as an additional non-primary, the zone settings re-appeared and he was able to restore the MX records. But he could have configured them from the registrar's web admin for the old domain.

I cannot understand why clients need switches?

Ones who get domain + hosting from same provider mostly don't. Ours is a fooking mess.