Let me stress this from the outset these are speculations.
I’ll start with some comments.
If there was an Atlantean civilization that fled East to Egypt and in search of higher ground, some of them might have carried on going and the obvious place to shelter from deluge is somewhere land locked and at high altitude like Tibet. The rising sea won’t easily get you there! Being at around 4000m on the plateau the altitude is similar to the Jungfrau Joch which would make it a great place to do observational astronomy / astrology. They may indeed hold a repository of age-old documentation. At one stage in history half the male population were in the monasteries. Knowledge and wisdom is / was taken seriously, it is not a frivolous land. The level of ingenuity needed to build some of the structures would have been world leading in its day. If people wish to preserve messages and knowledge in the days before terabyte hard drives, it would be nice if the essence of teaching could be carried forward across the generations. The detail could be written down, but it might need a teacher to bring it to life. If you observe, for example, Tibetan ritual magic it is of the very highest order. It is concerted, highly organised and rhythmic. Bear in mind there may be a whole lot more of this which nobody else gets to see. For me it is not beyond the bounds of possibility that they have / had developed a skill level unimaginable to me.
In general, a reincarnating lama or Rinpoche is recognised as a young child. Often, they are found by dreams, or by consultation of the oracle. The incarnation is then recognised by one or more high lamas. There can be tests of recognition of the artefacts of a previous lama. I am not aware of any middle-aged git being recognised.
In the system of raja yoga, nirvana means the blowing off of the casual vehicle, which can cause death, or the form may persist until physical plane death sometimes referred to as para nirvana. In the case of a fourth-degree initiate this would mean the dissolution of the mayavirupa shell in which the now liberated buddhic/atmic essence is indwelling. At the destruction of the causal vehicle the being would have no personality, no soul to speak of. There would be nothing of the former being left to recognise. Technically speaking they are no longer human, there is no compulsion to reincarnate because there is no longer any cause.
The status of bodhisattva refers to anyone who has committed to achieve enlightenment. There is no obvious way of determining how far along that journey they are. I am guessing that when the commitment is fully made it is referred to as sotāpanna. Having made a heartfelt commitment, the reincarnating jiva enters the stream leading towards Buddhahood. There may be a fair way to go. There are many academic discussions about the degrees. I doubt these discussions are made by buddhas. They are often of an intellectual bent.
At the third initiation the bodhisattva is well on the way and success is ensured. Such a highly evolved being may for the benefit of all sentient beings delay his / her enlightenment so as to return and teach. It is too late to do this once one they have removed the causal vehicle. The initiate of the third degree is still technically human. On a subsequent incarnation they could build a high-quality vehicle in which an echo of the prior personality exists and of course it is imbued by the indwelling causal vehicle. This might make it possible for someone clairvoyantly able to recognise the indwelling reincarnated essence.
Nobody knows with any certainty the level of initiation of the 16 arhats. When the texts says so and so got enlightened does that mean that they got a bit more enlightened or does that mean that the job was finished? In other words, the weight of the causal vehicle was removed so that they are less heavy ergo lighter. If this is the case, then a reincarnating fourth degree initiate is likely not to be recognisable because there is nothing of the old “self” left.
A being might have “qualities” pertaining to some historical figure, be reminiscent of and even express the essence of what the other being was. It does not necessarily mean that it is a reincarnation of the causal vehicle.
Given what I have read I personally believe that it is not impossible that these lamas and monks, who dedicate their whole lives to it, can train themselves to the degree that they can, with prescience, point at their own incarnation.
In Toltec terms the dreamers of mankind are group conscious. It is also possible that a reincarnating jiva can “talk” through the medium of dream to a close colleague and let them know where they can be found upon rebirth.
Someone caught up in the centuries old preservation of the essence of knowledge and as an act of service having tulku rebirths might indeed be acquiring karmic merit whilst putting off his/her own release into Buddhahood. Indeed, this sounds like a very bodhisattva like thing to do.
I’ll speculate further that the beings doing this are high grade disciples and initiates of the first three mundane degrees.