Tom, but without third-party software you can't do anything (except perhaps Expose) with the extra buttons on the MX900. Game pad Companion works for this purpose:

http://www.carvware.com/gamepadcompanion.html

And for non-Bluetooth mice, USB Overdrive also works (he's been promising BT support for at least a year now).

The program Dylan mentioned, Controllermate might work as well, but it looks extremely and unnecessarily complicated/confusing/awkward to use.

As for the first part of this thread: Replace the KVM switch and don't use the adapters with the mouse.

Bruno