The issue's been fixed. This is one of those times I wish I had some miracle solution to offer but the fact is that all I did was redo something I originally tried (point number 4 in my original post list of stuff). I retried the solution offered in this post:
https://github.com/modxcms/revolution/issues/12788
For some reason, it worked this time but not before. Note that core/cache had been cleared multiple times throughout this process, including the original attempts.
Computers suck sometimes.
