The two things in the subjectline are unrelated, I just want to ask about each.

If Punisher is on elevated and is looking down at say, a 3x3 formation of single base figures in clear terrain. If he targets the person in the center with Chaingun and hits, can he then roll eight other attacks after that? I had heard that there was a cap on the number of attacks, but I can't find evidence of this.

Also, the rules say that during a Duo Attack, you can only use a single target when doing it with ranged combat. If I'm based by more than one figure, can I do my two attacks on two different figures in close combat?

You didn't find any?!?!?

Well, that is probably because there isn't any. It's perfectly legal.

Yes. And the single target part doesn't mean that both attacks have to go against the same guy. It means that even if you have 2 or 3 's, you can still only target one guy per attack.

Uh-oh. Oops.

The ability's description is worded kind of poorly so it's easy to make that mistake.

For the chaingun, lets say he's firing into mystics. The description of the power doesn't say he stops making the attack rolls if he loses the power. Is that because he can take damage and roll another attack regardless of him still having the power, or is there another timing issue?

MYSTICS
When this character takes damage from an attack, the attacker takes 1 damage. This damage is not an attack.

So Punisher would take damage after damaging each Mystic, since CHAINGUN is a series of separate attacks and they are not all resolved simultaneously (like EE would be).

And if Mystics damage (or even critical miss damage) knocks him off CHAINGUN, the attacks are over.