My reason* for believing there is a significant kernel of truth to these theories of Jesus living in India and being buried in Kashmir, is that my master, Meher Baba, said that was the case.
However, Meher Baba was also clear that Jesus was the Avatar of that era. The Avatar is God descended into our realm of cosmic illusion as a man, the ideal or perfect man. Thus he is both God and man simultaneously.
The author of this website appears to have a bias (we all have biases) that Jesus was a man only, and that any suggestion that he was divine or of divine origin is a matter of myth. I think his thesis that Jesus's memory was embellished to make the new religion more palatable to gentiles who were already familiar with similar myths, is plausible. Again, for my bottom-line evaluation, I personally, as a follower of Meher Baba, turn to his explanations rather than to scholarship. (You of course are welcome to give your own weight to various sources.) According to Meher Baba, Jesus was conceived and was born "the usual way," i.e., the virgin birth was a myth added later on. Jesus did not die from the crucifixion in the sense we usually think of death, but what he did was also not something possible for an ordinary man--he entered the state of Nirvikalpa Samadhi and then returned after the crucifixion, and went to India. If you study the life of Sri Ramakrishna, you will find that he often entered this State, as for example when something would remind him strongly of some particular attribute of God, or when devotional songs were sung in his presence. Nirvikalpa Samadhi is the state of God-Realization or Self-Realization. Jesus, as Avatar, would have had this state along with his ordinary consciousness of being a man, simultaneously. Nirvikalpa Samadhi is only attained by the real death of the limited mind and ego. Jesus already possessed this state of Realization (having been unveiled from it by John the Baptist). So in a sense, Jesus died the real death on the cross, not the faint shadow of it that we consider to be death. One could also say that Jesus was already "dead" in a sense; or one could equally say that he was already beyond death and in the state of being fully alive, so that it was impossible for him to die in the usual sense.** In this sense, whether Jesus took up his same physical body after the crucifixion or didn't, would not change who Jesus IS one iota. He could take it up again or not take it up again and he would remain the Avatar. In the same way, he was the Avatar before his incarnation as well--"Before Abraham was, I am." The entire matter is really beyond words to describe adequately, and it comes as no surprise to me how 2,000 years of interpreting and trying to codify it into a rigid doctrine resulted in what has come down to us.
Meher Baba also makes no mention of Jesus being married, but instead refers to him as being single. Comparing events in the life of Meher Baba, the story in the Tomb of Jesus website of the king insisting on Jesus having a companion, and Jesus finally agreeing to allow a woman to assist him, is plausible. However, his having had sexual relations with the woman is not consistent with the behavior of spiritual teachers who have attained this degree of spiritual advancement. I would guess it was assumed on the basis of gossip, by people who could not conceive of a man living with a woman and not having relations with her. If you study the lives of spiritual masters who have achieved this state of consciousness, you find that some of them marry and some do not, but they never get entangled in casual sexual relations. While it is true that spiritual masters of this calibre may have a significant other, very often even this is a chaste relationship. Such was the case with Sri Ramakrishna and his wife Sarada Devi, and one may interpret that it was also the case with St. Francis (who Meher Baba said achieved this state of Nirvikalpa Samadhi) and his beloved Clare.
My main point here is that one must be careful not to throw the baby out with the bathwater. Though it may turn out that Jesus physically survived the crucifixion, and it may also be true that his history was embellished in the manner of myths that had also been applied to other historical figures, this does not mean that he was not an Incarnation of God. It doesn't necessarily mean that he was simply a prophet, a man, who lived through the crucifixion and then died later as an ordinary man. It also doesn't mean that there aren't important truths embedded in the myths that have been preserved as traditional Christian doctrine. It does mean that these myths contain distortion and embellishment, and that it will be a painful and disorienting process for traditional Christians to sort through it and find the truth of it. When the "dust has settled," I think they will find that Jesus was and is more than traditional Chuch doctrine has portrayed him to be, not less. (Just as the true goal of religion, God-Realization, is much more than living in paradise forever in linear time.) What will emerge, I believe, is not that Jesus was just an ordinary man; nor that he was the only son of an anthropomorphic God. Jesus will finally be let out of both the little conceptual box of materialism, and the little conceptual box of Christian dogma, in which boxes he has been imprisoned for centuries.
For more information on Meher Baba, see my article, "A Tapestry of Meher Baba's Connections with the West."
*Another clue is that when Jesus appeared to the disciples, he appears to have had a body with the same wounds. It makes no sense to me that Jesus would have resurrected a body with the wounds. It further makes no sense to me that Thomas would have thrust his hands into the wound in Jesus's side, literally, as per the Biblical account, simply because no-one who loved Jesus as the disciples did would have caused him excruciating physical pain just to satisfy their own selfish doubts. Therefore the account must be exaggerated (another clear indication of editing in the New Testament). Nonetheless, we are probably safe in assuming that this much is accurate, that Jesus's body bore the wounds from the crucifixion when he returned. Under those circumstances, it is a much simpler and more logical interpretation that he survived the crucifixion than that his body was supernaturally resurrected.
**My interpretation of the crucifixion, based on studying Meher Baba's life and teachings, is that Jesus was vicariously taking on the suffering of the world, so as to make the world's load of karma bearable for us. (Meher Baba said that the Avatar did not take on the karma of the world per se, which I interpret to mean that he took on the suffering that resulted from the karma but not the karma itself.) The external crucifixion of Jesus was thus an outward and visible sign of the real inward crucifixion, of the inner spiritual work that Jesus was doing for humanity and for all of Creation. Thus the real inner crucifixion was much, much more than just the outward event of the physical crucifixion.