I'd have a bit of trepidation including your father in law on the initial reservation and then trying to cancel just him. You are depending both on the clarity of communication and the skills of the agent. It appears very, very easy for a less skilled agent to trigger a reprice. You may well trigger one attempting to cancel one passenger.

Since an open sleeper reservation is a completely separate reservation, you are in no danger of triggering reprice on the original reservation by doing one. Yes, they are a pain, but if your father in laws participation is uncertain when it comes time to make the reservation, the safest thing to do is leave him off. You can always add him with an open sleeper ticket, which is a standard, if somewhat obscure, process. Cancelling one passenger on a ticket while keeping the accommodation reservation intact is not something I know about or have much confidence in.