For OSX, just create a user account for Limewire, and enable Fast User Switching (I test all the betas in such an account, which isolates it from my regular account). No regrets in the last year and many many betas. Because LW then runs in an isolated user space, all your other stuff is protected. Just use the Fast User Switching to let LW keep running in that account while you do whatever you regularly do in the regular account.

For the OS 9 mapping of mp3's to quicktime, the easiest fix is to open the QT Control Panel, and get it to reset QT as the default type. If that doesn't work, it's time to rebuild the desktop/ start messing with the TYPE/CREATOR codes/ searching posts in this Mac forum for "Quicktime"