OK, I figured out a fundamental issue that I messed up, now that I got that PHP error message. Since it was looking for a config_default file but not finding it, I realized that I misread the instructions and only renamed config_default to config_local rather than making a COPY of config_default and renaming that. Grrr!
I have fixed that issue, moved the COPS files back into the /Calibre directory and successfully loaded my library into a browser. It runs beautifully in my web browser on my PC. I had some trouble getting it to add on FBReader until I realized that for whatever odd reason, it was indeed case-sensitive there as well. Sigh. I am astounded at how fast it runs given that I have a huge library (24,000+). Yes, you read that right. Calibre on my desktop runs like molasses in January by comparison.
In the interests of dingbats like myself who just follow directions without really understanding what we're doing, probably step 1 of
https://github.com/seblucas/cops/wiki/Howto---Synology should also include to check the "Enable display_errors" checkbox. I still wouldn't have known to load checkconfig in a browser without somebody's help, though, and I really appreciate your assistance!