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.