The list is the source of truth, by construction
Trust in a record comes from where its data enters. Here there is exactly one door: the reply button on the invitation page. No message history is mined, no bot lurks in a group chat extracting maybes, no contacts get imported from anywhere. A guest exists on the list because that guest answered, which is why the count survives a call from the caterer.
Everything the day needs hangs off that single record: the number, the meals, the seating worked from arrangements on the same list, the reminders aimed at gaps, the arrivals at the door. One list, and the whole evening agrees with it.