Pashuteh Yid-
Your citation of the Rambam is very problematic, for there is more than one place where the Rambam says openly that one who denies ???? ???? ?? is in fact an Apikoros. For example:
1) Hilchos Mamrim 3:1 – ?? ????? ???? ????? ???? ?? … ??? ?? ????
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2) Perek Chelek, Yesod 9 – ??? ?? ????? ???? ?????? ??? ????? ??”? ?? ?????? ????? ??? ?????? ????? ??? ????? ?? ????? ????? ??? ????? ???? ??
3) The perek you quote from the Rambam comes from is loosely based on what he writes in ????? ??????? on Chullin 13a. However, there he is ???? all ?????? of ??????, no matter what. (I must acknowledge however that this is only according to the new Kafih translation, in the old Ibn Tibbon translation and the one found in the Meiri I have found the opposite. However I was told by my history teacher who is also a rav and a talmid chacham, R’ David Katz, that the only reliable translation is Kafih’s.)
Therefore I think your quote is much more a ???? than a ????. I am not sure of the answer but I would speculate that the Rambam needs to be understood in context. The only reason a non-Jew’s shechita is no good according to the Rambam is as a ???, for perhaps he will shecht ??? ????? ??? and render the animal ???? ?????. Therefore a Jew who is not ???? for this, even if he is a ????, may shecht and his shechita is kosher. One who believes in Torah Shebiksav but not Torah Sheba’al peh, might very well be an apikoros, but he is not ???? on avodah zarah if he does not deny Torah Shebiksav; thus his shechita is kosher. What the Rambam means is that only Apikorsim who deny both may not shecht, because they are indeed ???? on avodah zarah. But he did not mean to imply that someone who only denies Torah Sheba’al Peh is not an Apikoros.
Whether or not you agree with this pshat, I think it is clear from many other places, some of which I have cited above, that the general shita of the Rambam is not as you say.
I will say this though; I recently came across a Maharal in Gur Aryeh on Shabbos 31a who says explicitly that one who denies Torah Sheba’al Peh is not an apikoros, and he concludes that therefore the Karaites are not Apikorsim.