I tried commenting on there but it wouldn't work, so here is my response:
You start your post saying: "The purpose of this post is not to reveal whether Ibn Sayyad was the Dajjal or not since one can only speculate, but to point out how unusual the traditions were, and to shed a little light on what was actually going on in some respects."
And then you end it with: " It was likewise permissible to swear to Allah (say wallahi) that Ibn Sayyad was Dajjal."
You are being absolute with things that are merely probable and speculative. Just because the Sahabas held certain opinions doesn't make them definitive, since they do make mistakes. First of all there are multiple ad-Dajjal, culminating in one that is the actual prophesied anti-Christ. Furthermore, Jesus alayhi salam is the one who kills the anti-Christ. So I would refrain from calling a Muslim, a Sahabi, the Masih ad-Dajjal. He did not call people to worship him or anything of the like. We are commanded to have husnud-dhunn (good opinion) of the believers.
Also merkabah mysticism is not the only mysticism...in islam we do have Sufism and the similarities are not incidental. Yes, there are certain black magic elements that are disguised as spirituality that have crept in over time, but there are legitimate similarities between Judaic Mysticism and Islamic Sufism, because they come from the same Divine Source, like the Names of God which are central occur in both Judaism/Islam (Ya Hannan, Ya Mannan, Allah). Medieval rabbis wrote about how Sufis resemble the Prophets and sages of Bani Israel more than Jews do and that there are more saints among Muslims than among Jews, which was why they wanted to reform Judaism.
Not all the Sahabas are cookie cut the same, they are as different as diversity among people is today and Sufis would cite Ibn Sayyad had a certain spiritual 'state'. It is not necessarily that shayatin told him certain things, it may be that his spirit dominated over his body and as a result certain things became apparent to him. Mawlana Ashraf Ali Thanwi in his book on Sufism and Hadith explains the hadith of Ibn Sayyad and what they teach us about Sufism. I suggest you read that book since it will explain many amazing and unusual hadith and contextualize them.
Also, there is no way Ibn Sayyad would be practicing merkaba mysticism at such a young age. Again, you have to be open to the reality that there are people who have certain states, and you do still encounter them today. Sometimes children with mental disabilities show signs of getting glimpses of the unseen and it occurs even with non-Muslims. This has to do with the knowledge of the soul, which the Qur'an emphatically states, in Sura Bani Israil, 85: "And they ask you (O Muhammad SAW) concerning the Ruh (the Spirit); Say: "The Ruh (the Spirit): it is one of the things, the knowledge of which is only with my Lord. And of knowledge, you (mankind) have been given only a little.""