I'm a little unclear of how you move from Act to Act. This is what I thin k it is:
Act 1: End with either someone correctly declaring Klein's lair or Klein attacking three times.
Act 2: No idea
Act 3: No idea
I'm also not completely sure of what the Cabin Interiors expansion is even for but I'm guessing maybe learning how to end Acts 2 and 3 will fix that.
Also, once we're in Act 2 and 3 is there still a Klein Lair? Is it different or the same place? Is it still safe? Are there still reveal cards and peeking for Acts 2 and 3 and/or when you're in the Cabin?
Each Act can end with either someone correctly declaring Klein's lair or Klein attacking three times.
In each Act the location board gets tighter and tighter. The Cabin Interiors expansion adds different locations and changes the location board map so there is a possibility of being trapped in a location depending on if certain locations are eliminated during attacks. That is why the single use ladder is available allowing you to escape or get to a location that is cut off.
There is a Klein Lair in every Act. You must shuffle up the location cards and redistribute them to the sleeves. So it is most likely a different location but it could be the same as a previous Act.
Klein's Lair is always a safe location to be in at the end of an Act. Since Klein does not go back there to attack.