Let`s start here:
Antagonist is an adversary/oponent to the protagonist,for example in a story in which Lex Luthor is a protagonist Superman would be an antagonist,despite Lex being a villian and Clark being a hero.
In what way is Quinn an opponent of the Mc?She wants to fuck him,so what does she opposes?His virginity?That ship is long gone.
It`s hard for me to even see her as a villian,sure she acts outside of the boundries of law but so what?Is anyone getting hurt?Forced to do anything?No,she simply provides products demanded by the market.
To me she seems very much comfortable as VP.
She has the power already,it`s Sage that is nothing more than a figure-head.
Mhm,this is much more charismatic character:
"I got this" and Sage goes away.
We don`t know how much administration knows,we can be certain Burke uses the services,that`s all for now but what we do know is that Quinn made it work for at least a year before and Sage didn`t noticed.
""unrepentant villain," just... not a terribly effective one." - Quinn manages to achive what she wants,so perhaps the uneffectivness is simply her not being as villanish as some would like her to be?