Maybe I'm old-fashioned but if I was affected by this issue (and it had persisted for so long) I would set up an SMTP server and an appropriate MX record, then

Maybe I'm old-fashioned but if I was affected by this issue (and it had persisted for so long) I would set up an SMTP server and an appropriate MX record, then demonstrate using mail logs or port snooping that this was definitely not a receive-side issue. Of course the support organisation here (Google) should be well equipped to set up this kind of testing themselves and work with a customer to root cause the issue. In my experience though these issues are rarely taken seriously until there is no-one else left for support to blame.