If I had to guess what could possibly turn Redcloak, I'd have to say that this is where Jirix squishing the cockroach comes into play. Redcloak gave Azure City to the goblins because he wants to prove that they are just as capable of creating and maintaining civilization as any of the sentient races; if it's somehow revealed to Redcloak that Jirix went Full Self-Destructive Dictator immediately after Xykon and he disappeared, that might put just enough doubt in his mind to compromise--he would still have plenty of excuses for the goblins, but he might get just enough doubt to think that maybe a more cooperative approach to sealing the rifts is necessary.