Can one attend a conservative wedding in a conservative shul? A conservative wedding in a hall? I guess the answer is no. But what is the reason? Is it the same as praying in a cosercative shul?

HaRav Moshe Feinstein in various teshuvot [=responsa] (see for example Shu"t Iggrot Moshe Even HaEzer I 85) has explained that since Conservative rabbis reject some of the basic principles of Jewish belief and have abandoned parts of halacha one cannot pray in a Conservative place of worship. Their wedding services are problematic because the required two witnesses to the kiddushin probably do not meet halachic requirements. You can, however, participate in the dinner in a wedding hall (with an Orthodox kashrut hasgacha) after their ceremony if it is necessary for shlom bayit so as not to burn bridges with family and friends.