As far as I understand, the line "DMARC policy violated due to DKIM domain ("google.com") not belonging to the same parent domain as the from address field domain ("mydomain.com")." is stating that the email is originating from a google.com server, but the From address is in my domain; the two different domains are what's causing the DMARC to fail.
The response from unlocktheinbox states that all my tests pass; DKIM, SPF, and DMARC are all set up correctly.