It's sad about your disabled friend this is a really great game. Anyway, on to the questions!
1. How exactly does stun work? If I hit multiple heroes with a stun attack do ALL heroes get stunned or only the first?
All heroes get stunned
(rules don't mention this specifically but for example petrify DOES say only 1 hero gets effected)
EDIT: Okay the rules DO say this specifically, under Shot Modifiers page 12
1.5. Do stuns last the entirety of the room UNTIL a friendly hero comes in contact?
1.8. If three heroes or all but one hero is stunned does this mean all heroes are technically no longer stunned? I am very confused on this part in the rules regarding one hero left un-stunned.
The rulebook is talking about the scenario when there is only 1 hero left on the board (the others may be dead, devoured or teleported). Since it would be (mostly) impossible to set him free from stun, he is declared immune to stun. Otherwise, stun would technically be a insta-kill power, which would be too powerful.
2. When using the fire demon do you use the fireball piece that the wizard uses for the attack?
Yes (not the giant one with the red flame though!)
3. When teleporting another character must you flick and come into contact with them? or a teleport can function without contact anywhere in the room?
Nop, you can teleport anyone as long as the teleport spell has the Hero icon (see page 11)
Have fun flicking!