Both the WIN and MAC versions are there on the DVD. However, you may have to open it in OSX to see the MAC installer.
Yes, the same dongle works for WIN and Mac.
No, you can't use a USB stick as a system drive. USB is far too slow for this purpose.
It is possible to install OSX on a non-Apple built computer with Intel processors. It is a bit tricky and not all hardware works. There are plenty of sites on the net that you might visit for information. Just google "Hackintosh".
Cubase 7.5 - 64bit, WaveLab Elements 8, Mac Pro 2.8GHz 8-core 8GB, 10.9.1 Mavericks, Firewire Audiofile (to be replaced)