I don't think it was too implausible for Sofia to defect, that wasn't an unheard of thing for folks during those rare occasions when the iron curtain was lifted. I'd say there is a bit of melancholy when considering the fate of many of the characters who take ideological stands against Stalinism (e.g. disappearing into Siberian obscurity, or outright death). That being said, I agree with your point that the ending (as well as the rest of the book) is distinctly American. The characters the book occupies itself with around the Metropol tend to be the individualistic free thinkers idolized by the west, so it shouldn't be too surprising it ends with a bit of western sentimentality.

I think folks looking for a bleak or darkly comic tome were bound to be disappointed by this one though, it's definitely more a cozy comedy of manners and light philosophical romp. Not every book that takes place in Moscow needs to be The Master and Margarita!