What were the most crucial production mistakes that Warner Bros. and Joel Schumacher made?
The first and most obvious one in my estimation, was the decision to fast track the production by a year instead the usual three year intervals that were in-between the Batman films beforehand.
This video goes further into why that was disasterous:
https://youtu.be/Gm4s5tM6Jgo
The second issue that really killed this movie was the inability to get Val Kilmer back. I think that had he and Joel Schumacher would've been able to get along better and put their differences aside, then it would've added much more credibility to the film. With George Clooney now as Batman, it was even harder to get emotionally invested, when it was the third time in a row that the leading actor had to be changed. Simply put, Clooney's relationship with Chris O'Donnell's Robin feels hallow because we first saw that particular Dick Grayson meet up and be tutored by Val Kilmer's Bruce Wayne.