In my opinon, it's because alot of people say "I won't play if the changelog is not good" and sometimes it does not always please other people, so they wait in exietment, and tell everyone of their freinds about the game. So the word spreads faster and more ppl will be encouraged to remember the game exists instead of seeing a "lame or good" changelog 1 week before game is released. But that's just my opinon on why it's hidden.
Nope, if I had a complete changelog then I'd post it even a week ago (check the thread mentioned in the first post, and my post there, I wrote about some changes there already). But I agree that posting changelog possibly late is quite comfortable, because there is much less time to complain about it
Also, reality vs expectations looks like:
we are going to wipe the server data and bring a medium-sized update on Friday 01.05.2015 @ 18.00
vs
Waiting for big, fat changelog
There are more changes than I expected, whole list will be posted today or tomorrow. There are no game-breaking changes in perks, SPECIAL and overall balance so good builds should be still quite viable.