In addition to your claim, geemee_camille (it does say that on Stephenie Meyer's website), SM said that despite the other issues, she believes it is impossible to create Renesmee. They obviously can't have a human actor---no human child can be that young and have the mature expressions of an adult---so they would need to use CGI. SM hasn't seen a computer-created human that looks believably realistic...so far. According to her, technology hasn't advanced that far yet, but I beg to differ. It seems computers can do anything these days.
Either way, I think that any movie that is 90 min/1 1/2 hours is too short. They could make it two hours (most movies are around that mark, anyway), but if they stretched it any longer, they would have to make sure the plotline and events were airtight-entertaining. We don't want to have a Transformers 2 re-enactment (that film was 2 1/2 hours of CGI explosions---it was too long and needed some more plot line instead of just filling the gaps with action).
I wouldn't mind two parts/movies. Then it would even it out without making it overly long. It would give the producers more room and less of a rush and a decreased hassle to squish it all in. They could end the first part with Bella on the medical table, and have the dramatic beating of her dying heart as the ending. Like those cliffhanger/edge-of-your-seat/cheesy action-drama films. I don't know; I'm just spitballing here.