Even though I didn't hook up with her at the restaurant, I prefer proper relationship build-up first instead - I think you're exaggerating, to me it looks as drunk spontaneous sex, it happens, and it's not like MC is total stranger to her, he saved her BFF after all. Don't see how it makes her a slut by any definition.
Everything so far looked natural in a game and not out of a character.
Also by this moment you can already be out of Emme path.
And I don't get where did you find any hint of betrayal, I played it long ago and vaguely remember - even if you keep both paths open - when Malena admits she likes MC - Emme says that she is not offended and will be happy for them if MC chooses her, she is her BFF and she wants her to be happy, the same goes to Malena.