Hi, Square.
I believe that Ra’avad’s issue with Rambam’s ain lo guf was not whether HASHEM had a Guf, but whether someone who mistakenly thinks so would necessarily lose his olam haba.
The issue seems to be loss of olam haba for a belief that was mistaken but apparently prevalent.
This is what he objected to.
As for the need for Ikkarim and Rambam’s rationale for advocating them, this is a complex matter, way beyond the limits of this forum. I would recommend Moshe Halbertal’s “Maimonides Life and Thought” as well as Orthodox academicians Kreisel, Hyman, Seeskin, and Manekin.