About the line that moshiach doesn’t have to be alive:
Growing up, all i heard was how every generation has a moshiach and Chabad said their guy is the one that best fits the generation’s moshiach. So based on the chabad of my youth, once the Rebbe died 25 years ago there was a new candidate. So it’s a machlokes the chabad of my youth and current chabad if the Rebbe is moshiach. The chabad of my youth would declare that only christians believe in dead moshiachs, and anyone who died was clearly not the moshiach. The Chabad of my youth would say that the random wackos in yerushalayim claiming to be moshiach are better candidates than the “heintigger” (previous) rebbe.
Of course, the “Stump the rabbi” video says that all chabadskers believe the Rebbe is alive, just a question of whether or not to publicize it.
As for the gemaras that the moshiach can be dead, there is also a gemara that there is no moshiach. And the Rambam says moshiach can’t be dead. The Rambam is the pillar of Chabad halacha, so you can’t disown him for this halacha once it becomes inconvenient. the Rambam says moshiach is alive, just like Chabad said 30 years ago.