My husband and I saw this movie and we really enjoyed it. We thought it was rather funny and yet sometimes harsh.
I just talked to my husband about most of your points(about the thing about a bad portrayal to fat people because of the big splashes and the chair/seat breakings), and my husband made a good point of why they would put this in the movie. He said it was because even though Hal is seeing the inner beautiful of Gwyneth's character, it needed the reality parts that she really isn't small like he was seeing her, so thus putting those parts in the movie(I hope this makes sense of what I just said).
I myself also think Jack Black isn't great looking and definitely not Jason Alexander(YUCK)....but they needed guys like this because if they were the hot looking kind, they would've had dates no problem right.