1) Well, it completely locks up the system even the mouse (or sometimes it makes the whole system incredibly slow) although the bugs causing that behaviour were more or less fixed in the last version. It's quite strange if you are still able to move the mouse...

2) Ah, you're right, I forgot you don't have the options menu on MacOSX. It's your "Preferences..." menu, listed in the application menu on the screen menu bar. But from then on it should be the same.

3) I don't know if your imac has enough horsepower to share 6000 songs. I rather doubt it. Try sharing less files (like 1000 or so)