You wrote: "Rav Moshe Feinstein (Shu"t Iggrot Moshe Orach Chayyim IV 91) prohibits davening in a Conservative synagogue ... because of Conservative rejection of basic principles of faith and observance." Can you elaborate? What basic principles of faith and observance are rejected by the Conservative Movement?
While my intention in this forum is to explain and educate and not to find fault, unfortunately it is no secret that according to Conservative Judaism as it is today in the U.S., the use of a car on Shabbat is permitted as well as the use of microphones in the synagogue not to mention the use of electricity in the home. These actions are an outright violation of the Halacha and regrettably these are only a few examples. Halacha is the transmission of the Oral Law which was given to Moshe on Mount Sinai. Being indifferent to the Halacha is an abandonment of the principle of faith set down by the Rambam that the Torah which transmitted to us shall not be altered or exchanged.