You guys are right, of course, but I was just having this conversation with my local judge the other day and he referenced me to The Dissent card, which says that you counter a team ability "like you would a power", and Quintessence/Power Cosmic says powers cannot be countered.

So that, specifically, is how you can't Dissent Power Cosmic.

So, just to reiterate:

You can Thwart a PC/Quint Piece. It's not countering a power. It's countering a feat

You can't Dissent a PC/Quint piece. You outwit a TA like a power, which said TA specifically prevents.

Stupid Batman keeps dissenting my Ultimates TA. Whaaaaa!

Actually, it's specifically because the team ability says it can't be countered.

OMEGA EFFECTS: Before Darkseid makes a ranged combat attack, you can choose any or all of the following options for the attack: 1) Darkseid's range value is halved and his line of fire ignores characters and hindering terrain; 2) Modify Darkseid's damage value by -2; damage from this attack is penetrating damage; 3) Darkseid's damage value is 0; if the attack succeeds, put the target in any unoccupied square within Darkseid's range to which he has a clear line of fire.

Number 1: Can he use that and use outwit or perpelx?

No, those are not ranged combat attacks.