1. For USB 2.0 support you need to install the VirtualBox Extension Pack, which you can get from www.virtualbox.org .
2. Read section 3.10 of the manual, particularly the bit about creating filters.
3. Depending upon your Linux version, section 12.7.7 may also apply.
4. Although your mouse is USB, VirtualBox is probably seeing it as a PS/2 mouse, so that may be a red herring. The same would be true for a keyboard.
(Manual references are based on version 4.3.6 Edition, but I don't think they have changed for a while.)