They were different in the flashsideways, most of them just had to make one more change to let go. Their development in the flashsideways was incredibly important to their characters. I don't believe it was purgatory like a lot of people, just another plane of existence crucial in their development.
They aren't and weren't. Most of the mysteries were explained, just not all the mysteries were important. The character stuff was more important and that's just how the show worked. The writers have been very upfront about this for a long time.
It's not a cop out, it's the way they chose to tell the story. Which is legitimate. If you don't like that, then that's fine. But don't fault the show for something it isn't. Just say you don't like the show and let go.