At the risk of being tedious, I would like to reiterate my stance, which, it seems, is also the stance of a number of others on this thread.
1. None of the Rambam’s criteria for Mashiach apply in any way shape or form to the Lubavicher rebbe.
2. Although he may very well have been ben achar ben from Dovid Hamelech, so may I, yankel berel, sechel, CS’s husband and Bernie Sanders. To the best of my knowledge none of the names I have mentioned have incontrovertible proof as to their lineage. The proof that the LR is ben achar ben has been derived by Lubavichers. Give me enough time and money and I can come up with proof of the same value (i.e. worthless) for any/all of the above.
3. Even if he was ben achar ben (and even disregarding his non-qualification in the Rambam’s other criteria), he is still not a suitable candidate because most of Torah-true Klal Yisroel considered his views unacceptable.
edited – the rest required too many edits to post