Last night, I figured out why updating Mypy in my test environment made some tests fail: the capitalization of some types changed. This might have been easier to notice if the change markers were meaningfully aligned with anything, not sure what's up with that.
In any case, with that done, I have what I need to plan out the tests. For that, I have a rough idea of the kinds of types and situations I need to handle, and given those, I just need to fill it all in.
However, I expect that at some point, the test times will end up being so painful that I'll end up switching focus back to the generic motrfile repo, so I can pull out the plugin-bearing code ASAP.
Anyway, maybe I'll get on writing the tests after this, maybe I'll do it tomorrow. Either way, I'm going to wrap up now.