So how will Chabad be vindicated, if it turns out to be, say, R’ Chaim Kanievsky?
Because it will show that the campaign to convince Hashem to bring Moshiach succeeded. What that person turns out to be is completely irrelevant to that goal.
Indeed the fact that the whole frum world now believes and accepts that Moshiach is a real person, who will come literally at any moment, and that we should be seriously trying to bring that about as soon as possible, means Chabad has already been vindicated. Because not so long ago many people didn’t accept or acknowledge that. That was the essence of the opposition and ridicule to Chabad’s Moshiach campaign. I think most people know the name of the one who mocked Chabad, saying “They think Moshiach is a bosor vodom”. Nowadays nobody would say that.