Chulent,
Your rational for discounting the opinion of the Ashkenazi Rishonim is speculation. You are claiming that they were just self-censoring, but you have no evidence that or that in the absence of self-censoring they would have held Christianity to be Avoda Zara.
Is it not equally plausible that having more contact with Christians and possibly discussing theology with Christians, Ashkenazi Chachomim had a better understanding of Christianity than someone like the Rambam.