There are no stand-alone MP3 tag editors for the Mac that match even the most trivial program for Windows unfortunately. The best for Windows (reportedly) at this time seems to be MP3Tag. I have licenses for MP3TS and Dr.Tag which both had their strengths. Now I just use iTunes along with numerous scripts for tag management. The biggest thing missing from iTunes is the inability to work on "files"

For splitting MP3s there are a number of tools, but I have no idea if they affect the sound quality of the split portions. Might as well capture to WAV, split and then encode to be safe.