I bet my smug avatar doesn't help either ;-)

There is a very easy to miss door at the top of the first step of stairs after Geralt escapes his cell, just after you put out your first or second torch and sneak past/kill the first guard. The trapdoor is in the room behind it..

Wonder if they made it hard to find on purpose.

Just played through prologue both ways. Killing the son vs. letting him live. If you kill him the baroness is being tortured, otherwise it is the son you rescue...

Yeah, but it also depends on which direction you approach from. I killed the son but because I approached from a certain direction she was already done being tortured when I found her.