If my seat has my nameplate on it showing that I have purchased it, NO ONE has richus to be seated, even if I don’t show up until Adon Olam.
I have no problem with a person holding seats at kiddush so that his wife/kids have time to join him and be seated as a family. That does NOT mean entering the kiddush hall in advance of the congregation to reserve the seats. Unless we are sponsoring the kiddush or attending a simcha such as a Bar Mitzvnch , we don’t stay for the food, we leave after the Rav makes kiddush. We eat lunch at home, not shul.
I’m old fashioned, I think kiddush should be wine, kichel, grape juice and maybe some fresh fruit.