Is one be allowed to go to a large kiddush and eat many non-mezonot foods to point of satiation? Is one not obliged then to make homtzi and bench there as well?
At a kiddush, where some amoung of mezonos must be eaten (for kiddush bemakon se’udah), one must be careful not to reach satiation. Even if other foods are eaten in combination with the mezonos, reaching satiation will obligate a hamotzi and birkas ha-mazon (actually reciting birkas hamazon depends on his having eaten “four eggs” of the mezonos itself: see Birkas Ha-nehenin (Stern), p. 40).
Note that the common custom is to eat more than the amount of “four eggs” at a kiddush, and see Vezos Habrachah who explains several reasons why there is some room for leniency in this matter.
However, reaching satiation will nonetheless obligate benching, and a person should be careful not to ‘overeat’ at a kiddush.