To make it clear – I do not care about j .
He is NOT THE ISSUE here. the issue here is the late leader of habad.
We are only using the candidacy of j to discuss the candidacy of habads leader.
If RAMBAN says that j is proven as mashiach sheker because of his failure to realise nevu’ot hanevi’im, the same applies to all other wannabe’s who fail to realise nevu’ot hanevi’m .
RAMBAM specifically addresses the wannabe’s . And how we should evaluate them . They have to pass a simple test.
Did they build the B’h , or not.
If they did not, they are out. They are not mashiach.
So, according to both, RAMBAN and RAMBAM, your leader is not mashiach.
Period.
[Thats besides other potential problems, not discussed here.]