I did hear from the fine folks at iThemes, and their customer service team did a great job resolving the issue with the log in loop. And they explained again their policy about Sync. I’m still not completely happy with that as I spent several hundred bucks to get a feature that I need for 10 sites that I can’t even use without losing another feature that I use on all my sites.
But the support people were quite friendly and did do their job, so props to iThemes for that. I’d have rather have paid the rent then throw away money, but lesson learned. I need to ask more questions before clicking “Pay” instead of blindly trusting something will work as I hope.