I would imagine two years at least until they're rebuilt and ready to open. Obviously the end of the hall has buckled out from the explosion, but if you look at the video of it again you'll see the whole hall physically lifted up from the blast. I can't see that being remotely safe now, so it'll need to be totally demolished prior to rebuilding.
In terms of criminal investigation, I don't think that's a surprise again considering how quickly it went up.
It's difficult to tell how finished the slide tower was, but questions will certainly be asked about the speed the fire spread considering how close it was to opening. I wouldn't be surprised to see extra time taken to make design changes off the back of it too.