I'll play devil's advocate and argue that Redcloak isn't engaged in the sunk cost fallacy at all. In order to be fallacious the expected value of proceeding has to be less than the the expected value of changing your course. Moreover it's the expected value as Redcloak values it.

If the Plan succeeds he gets goblin equality as enforced by divine mandate. Even if he didn't have a pathological need to be right (which he totally does) he'd value that extremely highly. He'd also get all the things Durkon is offering anyway, so it's not like he's giving up much more than he already has in that scenario. He also considers the failure case, the current world is destroyed and The Dark One gets a seat at the table for next time, to be better than the status quo. Given all that the expected value of the Plan is actually really high. It's not clear that there's anything he values enough to be worth abandoning all that for. At least from his perspective.

The Plan can't work of course and the Dark One likely won't survive to the next world. But the only source on that Redcloak has is Durkon and Minrah who Redcloak has rational reasons to distrust. They're long term enemies with no evidence and a vested stake in stopping him. Their stated reason for negotiating at all (stopping the Snarl forever) is something Redcloak thinks is impossible. He has only their word that it is possible.