I'm not sure about your first question
re: 2 - the iPod can only be linked automatically to one computer at a time. That said, you can probably have it copy music from both the PC and the Mac using either (a) the option to manually manage playlists/songs (and just dragging everything to the iPod - this is a pain compared to auto-updating, but it gets around Apple's anti-piracy limit of syncing to only 1 computer at a time); or (b) use third party software to manage this
re: 3 - yes, you can use the iPod just like any external hard drive - just turn this on through iTunes and it should appear just like an external hard drive would on your Mac.