I agree with “Rational” which admittedly may not help Rational’s case. A few points. I grew up and was bar mitzvahed in a Conservative Synagogue – which for various reasons spawned a few well known people presently in the Orthodox/Charedi world. (For one, the Hebrew School and the Youth Programs were run by Orthodox people. This was in the 1960s, 70s and into the 80s).
1. If you ask the sheilah — there are poskim who permit attending a bar mitzvah, aufruf, etc in a conservative synagogue. These “heterim” are on a case by case basis. In other words, i am not sure i have seen anything in writing about it.
2. Back in the 1950s the C movement did present a threat to the O movement. But, now, the C movement is really in death throes. Thus, even though nothing has changed philosphically about the movement (other than it is much less halachic now than it was years ago) — i agree that it presents no threat. (Very few kids leave the orthodox movement to go to the C movement — or the R movement. They just drop out).
3. Someone raised an interesting question about a “C” synagogue with a mechitza versus an “O” shul without. Both may be dinosaurs at this point — but i heard a shiur about this around 30 years ago. The rav said in the shiur that he felt that you could daven in the O shul (off to the side) but probably not in the C shul because, as the poster above wrote, they were denying as a movement Torah MiSinai.