There was never any question of Alfonso Borbon Dampierre being named heir. Franco had many bad qualities but nepotism wasn’t one of them. When Alfonso BD married Carmen Franco, that’s when his realistic chances went down the gurgler. Besides Alfonso BD had no credibility with the military and didn’t speak Spanish properly. It would have been more likely that Franco would have appointed his right hand man whose name escapes me at the moment as his successor and the “kingdom without a king” ruse would have continued.
I am not particularly knowledgeable about that period of Spanish history, but, from what I read, Alfonso was indeed a Plan B for Franco and was seriously considered as a candidate to the throne. And it had nothing to do with nepotism, but rather doubts about JC's genuine loyalty to Francoism, which, in the long run, proved to be true, as JC, upon becoming King, embarked on a path of regime change that would immediately lead to the redemocratization of Spain under the 1978 model of constitutional monarchy.