where are the intercom messages stored and how long they would be stored for a replay?

Good questions, to which I don't have the answer. One would hope they are stored locally, and not for very long. I managed to record two that were repeated when I said "Hey Siri, repeat all intercom messages". A short while later only one played back, and after that I got the response "There's nothing to play right now". Perhaps you only get one or two repeats.

tt2