“The Rebbe was a descendant of King David.”
I have to admit that I was waiting for that one. Who said he was a descendent of Dovid Hamelech? He did, and his father-in-law did. I think that’s the extent of it. He may indeed have been a descendent, but claiming to be does not mean you are.
Furthermore, their claim to descendency (I hope that’s a real word) is through Rashi… who only had daugthers! Finally, although I don’t remember all the details at the moment, their claim is not through Shlomo Hamelech, and the Rambam, in Peirush Hamishnayos (Sanhedrin 10:1), writes that whoever says Mashiach will not be a descendent of Shlomo is “kofer BaShem uvedivrei neviav”.
“The Chabad Rebbe know everything in Torah that the Brisker Rav knew.”
How do you know that? The Lubavicher rebbe certainly knew a lot, but how do you know how to make that comparison?