For those curious in the future, I managed to solve this problem by doing two things:
1) Specifically add "index.php" to the end of my targeted redirect URL in Google's API console.
2) Specify as part of the Hybrid Auth providers configuration for Google the full redirect URL as entered in the API console.
I only discovered Hybrid Auth about two hours ago, and I hate that I had to go digging in the source to find that solution, but a solution is a solution...
Raymond