Does he have the power to take a middle road.
This is a surprisingly difficult question. Cheliax's protection of Razmiran comes from their (correct!) belief that he can be paid to support them in their inevitable-unless-Cyprian-dies war with Galt; they therefore consider him a useful ally amongst the minor powers. Razmir is not so ignorant in the ways of savage Avistan that he does not comprehend this; his diplomats are corrupt and mendacious, but having grown up amongst the barbarians they understand their ways, and they cannot hide their thoughts from the Living God. He therefore has an excellent theoretical understanding of the logic of diplomacy.
His practical experience, however, remains a series of bungles and failures matched only by all the other low-WIS, low-CHA people who have managed to conquer countries by being Just That Badass, mitigated only slightly by Detect Thoughts. Razmir does not know if Him supporting Lastwall will so greatly offend Cheliax that they decide he is better off dead; they probably cannot kill him - they repeatedly failed to kill Felandriel Morgethai during the Andoran war, and her school is much more accessible than the set of demiplanes that is where his power lies, and which he never leaves save to one of the most defended fortresses in his kingdom. Perhaps they could throw a sufficiently overwhelming quantity of eighth-circle spells at him to drag him from his plane-nest, or invent a sufficiently cunning scheme to lure him out of it, or assemble a sufficient number of adventurers and constructs and bound outsiders to destroy him when he ventures outside. But these seem implausible to him.
But they could take Razmiran from him. Or, to be more specific, Cyprian could take Razmiran from him, if he was willing to accept the prospect of Razmir throwing all the offensive firepower a ninth-circle can provide at making Cyprian regret it. Razmir has the power to destroy cities from the safety of his demiplane, but he does not have the power to hold territory, collect taxes, or demand worship without administrative tools that Galt can shatter.
... In which case he does not need Cheliax, he merely wants them, and an... alternative patron... would be valuable. And - if an alternative patron would be valuable - than Cheliax will have their chance to bid to learn of it.
Unless they believe he is a firm ally of Lastwall, armed with the power to conquer death without diamonds or the vast expense of Clone, in which case destroying him becomes an absolute necessity. Asmodeus can destroy him, even if Aspexia Rugatonn cannot.
And if he does not have the power to betray Cheliax, than where will his plans for ascension go? The time will come when Asmodeus realizes that Razmir is a demigod in truth, and then he had best be able to make himself beyond Asmodeus's power to easily destroy.