I'm positive Bradley didn't know all along that it was Stacey, but she may have told him in the bedroom scene during the live episode. That makes sense to me 'cos it would explain why he told her to run just before he fell.
I've no idea if Becca knows, but I'm inclined to think not 'cos she couldn't have kept her mouth shut about it. Also why would Stacey tell Becca when she hadn't even told Bradley?