it seems that service by first class mail along with proof of service may be sufficient.
It is, if it's accomplished by a non-party. In Ca, to renew a judgment, the creditor must do three things.
1 - Apply for the renewal using form EJ-190.
2 - Send notice of renewal on form EJ-195.
3 - Complete the proof of service affidavit on form MC-012.
According to the second page of the form, service may be made by mail or by personal service by a non party.
When serving by mail, you are correct that service is complete upon receipt of the notice by the US Postal Service.
Is the renewal still valid if I wasn't served, and/or would it be a valid argument to present to the court that I was never served?
See Page 2 of form MC-102:
http://www.courts.ca.gov/documents/mc012.pdf
Then go to the courthouse and look in the case file to read the Proof of Service on the form MC-102.
The renewal is valid if the form is properly filled out even if, for some reason, you didn't receive the renewal notice.
You can, however, go ahead and file to set aside the renewal and claim that the Proof of Service is fraudulent (if you believe it so) but I suspect you'll have a hard time proving that.