@Slominer,
“How many Lubavitchers left Lubavitch after (because of) the Rebbe’s petira?”
I don’t know of exact numbers, but it was a handful, individuals. There was no mass exodus.
“Why did they leave? And where’d they leave to? You both say few of them left to other Chasiduses; so where did they go?”
Most of them left Yiddishkait entirely, perhaps because they never really believed too much to begin with, but the Rebbes presence and the environment that created left them within Lubavitch for the time being.
“And how did the Liozna Rebbe and that Belzer Dayan explain their leaving?”
I don’t know their explanations, you’d have to ask them. I can tell you that as early as 1987 this Liozna Rebbe was expressing his doubts in Lubavitch and the Rebbe. He only got up and left after the Rebbes histalkus.