CharlieHall,
Again, to determine the issue whether a Jew may enter a mosque (which as you said earlier is the question at hand), it seems to me you must first determine whether Islam is considered avoda zora for a Jew. It is, as I sourced from the Ran, Tzitz Eliezer, Ritva, and Radbaz. And Rambam’s statement regarding it pertains how it applies to NON-JEWS insofar as the Sheva Mitzvos Bnei Noach are concerned in regards to idolatry. It is not only the majority opinion (that Islam is avoda zora to a Jew), but it is lchol hadeios. As far as the question as whether one may enter a mosque, offhand I am not aware of anyone who addressed that specific point other than the Tzitz Eliezer, who said it is assur. Are you aware of any other posek who directly addressed entering a mosque?