Google Music doesn’t upload all of the mp3s – how can I troubleshoot what the error is

Not yet.

You can visit google.com/takeout to see the services which Google currently lets you export your data from.

You can also visit the "Data Liberation" website to see Google projects where data can partially be exported.

However, neither of those list Google Music at all, and a random comment says it doesn't work yet, so the evidence is that no, you can't.
_{(Not being a US resident, Google Music isn't available to me, so I can't say for certain.)}

To update your ID3 tags on Play, you will need to set the codepage for your local MP3's first and then use something other than Windows Media Player to edit the tags in some way. A batch-edit tool might do the trick, and would make it easy to reverse the change afterward if needed. Add something to a field that you don't use, and then strip that change back out after the first update is successful.

If editing a tag does not trigger an update, you may need to delete entire tracks or albums from Play and then re-upload them with the new ID3 tags. In researching a related problem I found the delete-and-upload method to be effective and sufficiently quick, but your mileage may vary.