No, the Dalai Lama is a high ranking figure of one branch (Gelug) of Tibetan Buddhism, but not the leader of that branch nor of Tibetan Buddhism. One of the reasons he’s so revered by Tibetans, and so influential among them, is because the Dalai Lama, for centuries, has been the political ruler of Tibet (it’s king, more or less).Nominally anyway. In practice this has often not been the case.
As for the Dalai Lama being the “spirit possession”, or whatever, of the Buddha….honestly, I’m not even sure what that’s suppose to mean, but it’s not a doctrine I know of.
I will say that Tibetan Buddhism has a lot of quasi-mystic elements to it that aren’t found in other branches of Buddhism. Much of this is influence from the Bon religion that dominated Tibet previous to the introduction of Buddhism there. Buddhism by its nature (for instance neither claiming nor denying the existence of god/s) lends itself rather well to syncretism. It is quite easy to meld Buddhism onto the beliefs one already has (hence, for example, myself, an atheist Buddhist). So as Buddhism spread across Asia people tended to flavor it with their own native beliefs and practices. Even today we see this as Buddhism in the west has begun to take on flavors of its own.