It's a movie... *possible spoilers*

It isn't meant to be entirely realistic. Of course, every film should have some semblance of reality within its contained universe. Harry Potter can't simply do ANYTHING he wishes. Within this film I don't think it was so far out that the entire community went along with what happened. They were very tight-knit, all went to church together, and they all seemed to know Lars or his brother/sister-in-law well enough to care. As for it being an inaccurate description of mental illness, well, I wouldn't say that. Look at Criminal Minds and CSI, or all of those television shows that inaccurately display mental health in order to sponsor their killer of the week - people buy into it. This did a better job of portraying his condition than any of those. Lars was generally disturbed and his doctor's telling them to go along with it isn't wrong. Sometimes if you attempt to break a delusion you only end up causing more problems and worsening the situation and health of the patient. They weren't meaning to go along with it for the rest of his life; she knew that she was reaching out to him during their therapy sessions and was insistent that he be able to break out of it on his own. Is that so hard to believe that they'd rather do this than lock him away and put him on meds that would alter his consciousness? Now, some of the people getting so into helping with Bianca was a tad out there - I don't think they'd actually come to think of her as a real person as they did in the film. Still, the message of the film isn't about portraying things as realistically as possible but the love they put into helping Lars. I think the movie is great - good writing, great acting and direction. You don't have to like it, but try to come up with a better reason than it was too idealistic. Sometimes movies are idealistic to get their message across. It just depends.