Why would Pam work with Cliff against Bobby and Christopher? I already have a hard time believing Cliff would do it but Pam? No way, not in a million years..
Even if Victoria Principal considered returning to Dallas, I'm sure the moment someone suggested "You'll be playing a megabitch version of Pam out for revenge who orchestrated her son marrying his cousin under a different name!" she would have slammed the door in this person's face and that would have been the end of the negotiations.