Answers:

1) Yes. And no. In fact, no. The "Reset Metadata" function does, in fact, remove those items from the empeg's database. However, when you next connect using emplode, you'll be prompted that these items are missing this metadata, and you'll be prompted to fix it. Answer yes, and it'll all get sorted out.

2) They're done in the empeg database. The MP3 files will be untouched.

Anyway, like I say, once you reconnect with emplode, you'll be prompted to put these tags back (into the empeg database). This time, however, they'll have the correct values, and should avoid the "invalid file" problem.