All humour is based on confrontation – with the ‘barb’ taken out or the edge slightly blunted. Describe any situation that is potentially dangerous or embarrassing and you have a subject for humour.
This explains why humour changes because of culture, age, sex and intelligence, and why the difference between a cold or uproarious response are often so close. The more confronting the joke the greater the potential reaction. That’s why if you get the giggles at a funeral or other important event it is going to be harder to control.
It also encapsulates your theories but explains why incongruity is funnier if it involves greater embarrassment. I have a lot more evidence but not time at this moment.