In order to prove the Medusa that showed up was fake and not the real one, Uriel had but one choice: to find the original and rescue her from the skinwalkers. The Medusa that came with Sam said all the right things, was immune to silver and she was exactly the same but Uriel knew in his heart that it couldn't be the real one precisely because she came with Sam.
If she truly wanted to go back to her family, which was unlikely, she wouldn't rely on Sam for that. Not because it wasn't convenient, but she just broke up with Uriel and he knew she wasn't as cruel as to do something like that.
Skinwalkers, no matter their rank, were able to mimic every aspect of a human being. They were even able to perfectly imitate human emotion, but they didn't know how to act on those emotions because they were unable to experience them. Since doppelgangers used to be humans they might be different, but they too were allergic to silver.
