1/ Use N800 as a headset : not possible yet BUT you can dial your phone with phonelink and take your call with a bluetooth headset : phone stays in the pocket !

2/ You should be able to do that : use the GSM modem (but you'll have a 9600 bps connection ...) and you might have to pay for internet connection (you have to find a number to call ... depending on where you are ... if in France I can help but otherwise ...)

3/ You have to install a VNC server on your PC ... You'll find tones of HowTo by googling : VNC on N800 is just one vnc client among loads of ...

Hope this helps
Fred