yankel berel, your suggestion that the Ramban differentiates between someone who has started the process and someone who has not is something you invented because you were ‘concerned’ how the Ramban got away with his refutation when, according to Rashi, Daniel could be Mashiach despite having died. But if you look at the gemoro you’ll see that it has nothing to do with Rashi as it is the gemoro itself that says that Daniel may be Mashiach.
If anything, Rashi makes a xian view even less acceptable, because he says either Mashiach is someone who is alive and similar to Daniel, or it was Daniel. It is only by ignoring Rashi that someone can wrongly extrapolate and say that someone else who has died is Mashiach.
So why come up with a differentiation based on conjecture and for which you have no source?
Furthermore, I would venture to suggest that if anything you have it back-to-front. Wouldn’t you agree that it is more likely for someone who has started to process to be an acceptable candidate after death than someone who has not started the process? If it has to be the latter, then why yoshke and not someone else?
And that would make the Rambam even more simple to understand. That is, even though Bar Koziba, whom Rabi Akiva supported and claimed that he was Mashiach, was disqualified when he died, how much more so someone who did not ‘start the process’.
As to your claim that by accepting petitions and the like the LR is believed by Lubavichers to have ‘started the process’, since when is accepting petitions a criterion? It certainly has no source in the Rambam. At any rate, what Lubavicher do or do not consider in regards to their rebbe is irrelevant to us.