I never noticed the shift problem but I get it as well, If I get some time I'll try to figure out what is happening. As for the mouse buttons, it's a known issue and one that annoys me but I can't figure out a solution for. The problem is that mouse buttons beyond the initial two and middle one aren't actually understood by Windows XP so your mouse driver software has to inject them. The software handles injected clicks different from normal clicks so it just let's them fallthrough to the OS.

As for a Mac, unfortunately I don't own a Mac so development on a Mac version is at 0%.