What is the most amount of calories ever eaten in one sitting by a human

On a point of order, the gluttony cited is obscene but most of these examples relate to volumes of food consumed. A substantial portion of these meals is water. The OP question was calorie intake.

A bit over a litre of olive oil has 10,000 calories.
If you were to “wash down” with a (Imp) gallon of edible oil alone you’d have consumed 45,000 calories and probably on a gurney somewhere leaking calories from every pore and orifice.