Either I am misunderstanding you or you misunderstood me because I wasn’t clear.
The second.
For us though, he is a mi’ut (possibly even against himself) and we have to follow the consensus, not the individual opinion.
That is a different question. However, since there is no Halacha Pesuka (AFAIK, SA doesn’t mention it) one can “hold” whatever they want. To call reincarnation an “Ikar” is difficult to say the least.
P.S. That is not to say that it doesn’t exist. The Sefiros are also a strong basis of Kabbalah, but no one will tell you that you must believe in them or be a Kofer.
What I meant about the Rambam not having Rishonim to hash it out was that, had the Rambam been born now he would be like us, subject to the consensus.
I’m Moche on this as well (but this obviously didn’t come out the way you wanted it to, so I’ll just ask that you reword it).