I can't import mp3 files into iTunes

No mp3 files from any source, including iTunes itself, or just some particular batch of mp3s files from some unspecified source? It isn't unusual for some encoders to produce mp3s that are not compliant with iTunes. You can try using the application MP3 Scan+Repair (http://triq.net/mac/mp3-validator-mac-os-x) to fix them but sometimes you have to re-encode them. Try playing with Quicktime Player instead and report if it will play the files.

Oh, it also isn't unusual that files do not have labels built in. They are adding to iTunes but not ending up where you expect them to be.

Despite many conspiracy theories, iTunes is probably not designed to randomly delete music. The only way I know of for iTunes to be responsible for quasi-deleting content is if the library file is corrupt which sometimes happens when a new version is installed. Even then the media files are still around but their entry in the library has been truncated.

I don't see that iTunes Match would have anything to do with it unless you're talking about your library on your phone or something.