That wasn't Ven's heart, that was Master Xehanort's Keyblade. Besides, Ven would have become a Heartless and Nobody if Master Xehanort stole his heart, and Heartless/Nobodies will not appear in BBS.
Not necessarily. You only get a heartless when your heart is swallowed by the darkness or when a heartless takes it. If MX took it, seeing as how he's not a heartless, Ven probably would have just died. ;