It's a shame that certain parties did not respect your wishes for the goodies to not be distributed. In the past, I used to be one of those who didn't really care that much about the sharing and crediting, but I realized it's a proper gesture of courtesy to credit and to ask for permission before you share something that's not yours. So I pretty much understand the move here. And if you have to watermark to prevent the thieves, then by all means watermark the thing to DEATH.
I don't really mind, and if anybody wanted the original scan/art/work, they can ask them from the owners themselves (and you already know the people you have to look out for, so that should be okay).
About the locked forum for members, it's not a problem for me, because I'm logged in all the time.
If I might point out one downside to this, locking the forum will cease exposure of this forum to a lot of lurkers out there (the good ones, mind you) who want to follow the developments and posts. You might lose these fans halfway. But I guess if the fandom is really important to them, they will find a way... and register
It's not hard and it's a wonderful community here so it should be fine.
It's a good idea as posted by the previous users about the major updates links. You could have a "major updates announcement" up at the front that shows what's happening. That should be a good way for the members to log in when they see something they immediately want to see.
I hope the issue settles soon enough.