DrByte,
Update - I just got it to work with just a little more trial and error. Here's what I did:

On the 2Checkout Admin under Site Management I used the 'Header Redirect' under Header Return and in the Authorized URL I used:

https://example.com/index.php?main_p..._2checkout.php

The readme says to use the Direct Return option and assumes that ZenCart will do the rest.

Mine may have been an exception, but wanted to post here to hope we can make it easier for someone else later.

You guys are still the best!!

Good luck to all!