Considering that they were headed into a civil war when the Vietnamese invaded because of the Khmer Rouges policies... they would not have lasted much longer. Even if Pol Pot has that brief bit of sanity and refrains from attacking Vietnam... The Khmer Rouge would still be attacking Thailand, and Laos. Simply put the Vietnamese were just the first, and most powerful, to lose their patients with the complete nuts across the border. You also have the Eastern Regional Purges, which allowed the Vietnamese to build up their post war government and a native Khmer assault force. One burst of sanity is not enough to save the regime, although the aftermath would be different i cannot see the Khmer Rouge hanging on for that much longer then they did. They were non functionally insane on top of being a clear threat to all of their neighbors.
Well the Eastern zone couldn't fight the center. The only way those soldiers survived was by being able to cross the border, and they only started fighting when they were already decimated the cadres and generals refused to believe they were being purged until more than half their officers had been executed and their leader Phim thought it must be a mistake and tried to arrange a secret meeting with Pol Pot to straiten things out even after 400 of his officers had disappeared. The eastern zone made absolutely not moves to over through the government, they only fought because they when they started being slaughtered.
If the Khmer Rouge had not fought in Vietnam I think they could have survived a couple more years, their system of zones combined with the center allowed them to direct zones to purge each other which happened multiple times without resistance. Their was no organized resistance to Khmer Rouge rule by that point or really even unorganized resistance. Absent outside intervention I see them holding on for a while more, but I think at some point society might just collapse, in three more years you'd have an over 50% mortality rate, and I wonder if they could survive that... But I think they might and then what at some point something will break, but given how bad it got IOTL without internal dissent. I don't know, they were using more and more teenager and child soldiers even as leaders of cooperatives which led to obedience and more violence at some point it has to give, but when the whole country is a concentration camp, well it's hard to fight back.
I see what you mean about Thailand and Laos, I guess you'd need them to see how weak they really were, and if they knew that they wouldn't be who they were, still even for them, 45,000 light infantry vs a 1 million strong army with jet fighters tanks and rocket launchers... Even for them it's just math.