Yehudis – if someone tells me that they think rav moshe feinstein was moshiach, I don’t have to disprove them – i have absolutely zero reason to think that he is. I have zero reason to think anyone today or earlier is/was moshiach. It really doesn’t matter who moshiach is, but putting the lubavitcher rebbe in that equation is effectively saying “well, you can’t prove that he ISN’T, therefore he might be, and really what I’m saying is that i think he is or is likely to be, and you can’t prove me otherwise!”
It’s like saying someone believes in the tooth fairy. Upon questioning, they remark – “sir, you’ve shown me that there’s no oroof to the existence of a tooth fairy, but….can you prove that there isn’t one?” That’s not how logic works. We assume the non-existence of unknown things until they are proven at least to some degree.