Thanks for suggesting I start over, it worked, though I had not planned on completely starting from scratch.
After deleting the Rom slot and rebooting, I again recreated the blank Rom slot, then booted back into recovery, reactivated the stock rom, backed it up, rebooted into recovery and reactivated the rom slot, restored my backup, only to have the exact same problem as the first time. So, I decided to do a factory reset on the rom in the rom slot so as to get a clean rom to start over with, but even though I had the rom slot activated, somehow the result was to reset my stock rom back to factory and of course wipe all my data! Oh, well, nothing like starting from scratch with an almost virgin device (except Safestrap was still working just fine). I backed up the resulting nice clean rom, then restored it into the rom slot, and all went well from there, rooted it, added XPosed framework, HDXPosed module, flashed the Thor-Gapps zip, and now I have both Amazon services and Google play store and Google apps working side by side! Ya hoo! Thanks to all the devs who worked on this (and I will indicate the thanks in all the appropriate locations).
One question - should I also go back to my (now nice and clean) stock rom and root it and install Android firewall to attempt to block OTA updates? Would an OTA update wipe out Safestrap and the working Rom slot?