Follow-up: If the person would remind himself of the need to make Havdalah during the seuda hamafsekes erev Tisha B’av after he has eaten less than a kezayis of bread, he cannot bench now – since he has to yet a kezayis of bread. He cannot continue eating until he has eaten a kezayis since it is forbidden to eat before Havdalah (once a person has reminded himself of the need to make Havdalah, he cannot eat any further. The obligation to make Havdalah remains in effect until sunset on Tuesday, and if one is able to make Havdalah (he has wine and he remembers that he needs to say Havdalah, he is not allowed to eat prior to doing so).
He cannot make Havdalah during the actual seuda since it is forbidden from the Talmud, and forbidden from the letter of the law, to drink wine during the seuda hamafsekes (unlike during the Nine Days, when refraining from wine is a mere custom).
Please investigate and let me know if you find anything.