Update: I was able to reset the device with a paper-clip and restart it with it connected to the PC. It booted safely with no PRS+/Zork customizations installed.
I wasn't sure of the best thing to do next. I figured the custom build was still on there, and would remain there even if I did a hard reset, so I went ahead and installed the old Zork 650 safe build and restarted the reader when disconnected from the PC. Unfortunately this did not resolve the issue. Instead of a boot loop, the reader hangs on the opening book page (the bar is at 100%).
So I reset with a paper-clip again, connected to the computer and the reader activates with no PRS+/Zork customization active. Thanks to Kartu's robust coding I still have an ereader. Thanks Kartu!
In any case, I need to get the Zork build off. If I restart it, it will hang again when the Zork build kicks in. I'm not sure how to get it off. Can someone give me some suggestions please. I think I've been pretty lucky.
I don't know why my ereader borked on the old safe Zork build. Probably it was a result of having installed the untested custom version first. In any case, the safe version does not work as a rollback from the 650 chess version. I highly recommend the author remove the 650 chess version. I did everything by the book and I don't believe this is result of anything I did wrong.
I will leave my ereader on until I can work out how to scrub it clean of all custom firmware. Any advice would be appreciated. Thanks!