The short answer - No
The long Answer - May be
To do so would require very heavy modification to the point that the xbox is nearly two xbox's in the sense that it would need multiple GPU output.
Yes you can play up to 4 players at a time on one console while online
yes
It will be, yes.
yes
Yes, if you have the Halo 3 PC version of the game
They stoped online play for Halo 2 back in June 2010. And you need Xbox LIVE to play halo 3, however on the new Xbox 360 u can play halo3 online for free
you play online
some might get bored of halo and play w.o.w, but only if they don't have online halo 3. they also get bored of w.o.w and start playing halo 3 online.
No, halo 3 forge is not offline, you may play it online with friends or others.
no
yes alot of poeple still play halo most are mlg or pros througe
yes everything except multiplayer
Yes, Halo 3 can be played in local co-op campaign and in local and online PvP multiplayer.
Xbox live yes