So just to set the record straight: While the email issue still baffles me (but seems to have worked itself out), it turns out the iTunes issue was to some extent my own fault–an artifact of creating a separate library for video on an external drive. Once I figured out what had happened, it was relatively easy to fix by recopying my media folder and doing a global find and replace in one XML file.
This sort of makes me wonder (again) how people who are afraid to go under the hood of their computers and do things like edit XML files get by sometimes. But the answer is that they probably just don’t get themselves into situations where they would need to do so in the first place.
Anyway, all my music lives where it’s supposed to now for the first time in years and has been pretty much de-duped, etc. All I need to do now is fix lots of dates and determine which “protected” items from my ex’s account that were never converted to DRM-free versions I need to re-acquire (not many, it seems…).