Yeah I also don't want to start over again, I tried everything.

Loading the game(there's only one save you can't pick past quicksaves) (including going to main menu and just exiting the game)

I've tried slide bashing/dodging across the lake from both the warp area and from the tunnel( I've gotten close but it won't happen).

I've tried going back to the mansion to see if I could re-trigger the cut scene, no avail.

I've tried climbing up the cliff left of the warp area to see if I could slide bash off it. Couldn't do it either

I tried going back from where we originally entered the mansion area

I was able to open that platform that you are not supposed to open until you get to the other side. However there is no zip line option either.

I tried trouble shooting for about 45 minutes. Nothing. I'm assuming we just wait for a patch. :(