It's called tethering, and the reason everyone suggested 3G instead of wifi is because most laptops have wifi embedded so the market for such setups is mostly dead.

When mounted as USB, devices work as modems, not as LAN cards. However, you should be able to route the data normally, from one interface to another, though I think that could break stuff.

Your best bet thus far is to look for Maquarade, that should do the trick, only problem is I'm not sure you have the interfaces to route correctly, as the modem isn't really a network card.

Could be done. Some hotspot sharing software like the joiku or what's its name could route to a modem interface.

Also, explain all you want, it's an USB device not a magical fairy, the OS on the other side must know how to use it. It's never going to be an unshielded twisted pair. As a result you need the equivalent of the USB dial modem driver or software, regardless of platform.