I think the cast had known for a long time and were just not allowed to say anything until the press release. Most of them would have gotten over their shock a long time ago.

I agree with you, at least for the first season. The plot is fairly straightforward and could have been easily streamlined while only making small cuts and changes While there were obvious large changes to the characters that I didn't agree with I think one of the main things that derailed the series was making the Aes Sedai such a huge component of the season. In the series the Aes sedai start off as this mysterious organisation that is not fully trusted. It is not until the girls go to train we meet them to a greater extent and We realise how out of touch ad self centred most of them are.