You are being played by your heroes, mate.
I can accept that your metagame is different than mine, and that you think plot cards are more valuable than I do, but that's a hell of a house rule you got. No way I would have accepted such deal, even with booze involved in the bargain.
Firstly, unlimited currency supply makes no sense, otherwise like the above poster said you may just purchase your whole deck with no notion of saving said currency for it. Your crazy +12 threat per quest completely tears this rule apart, tramples it with iron boots and throws the remains into the fires of Mount Doom. But even more importantly, what do you do with this billion threat tokens? Play enablers and weak versions of OL cards in exchange of extra actions and rerolls to the heroes? No wonder why your heroes are okay with that. It's Vegas, baby [for them].
I realize that you have only played a few quests with this rule, and maybe you are putting a lot of faith into these plot cards. I don't think you'll find what you seek but maybe you'll have tons of fun doing it your way.