If one forgets al hanissim in bentching on Purim, there is a basic shayloh if the person has to bentch over again.
(It depends on the shayloh if one has to eat pas on Purim). Now, if one’s Purim seudah went into the night, one still has to say al hanissim. But here, if one forgot to, one definitely does not have to bentch over.
If one davens maariv before bentching at the end of one’s seudah, there is a shayloh about saying al hanissim in the bentching. The Mishnah Berurah suggests that if one does not want to stop the seudah, but he wants to daven ma’ariv (e.g,he wants to daven biz’man, or there is a minyan now), he should bentch, saying al hanissim, daven maariv, and then—just keep on partying well into the night! (this ain’t me–it’s the Mishnah Berurah!)
If one starts their Purim meal before sunset, and it ends after nightfall (this is not unusual on Purim), one still adds al hanissim to the bentching. If, however, one has davened ma’ariv already (before bentching), then al hanissim is not added.
If one still feels the effects of the meal, he is still not at all hungry, he may bentch as long as he does not feel hungry.
If the levity on Purim is done with the intention of it being a simchah shel mitzvah, it would be permissible (within reason) to do that in a Shul. If one is simply “having a good time” , then one runs the risk of violating kedushas Beis HaKenesses, a serious aveirah.