the issue here is not bending lengthwise - if you are getting newton rings in the middle (and those are newton rings, I think), then it's width-wise bend.
Weights on the end of film, rolling backwards, etc, should not do anything for width-wise roll. Again, the best way to dry flat is in a humid environment. Running the shower for about 2-3 minutes before you hang to dry works to decrease dust, as it pulls it down and out of the air, and increases humidity. That helps only a bit for width-wise but a lot for length-wise.
I put my film in printfile sleeves, and then under a book or two. It's difficult to keep an entire room humid but that would be the only way to really help the book flatten them out completely.