Yup! There’s a neurotransmitter for registering the compounds that trigger that hot sensation, and that neurotransmitter gets depleted over time, based on exposure. I don’t know the exact timeframes, but it’s on the order of days/weeks.

https://en.m.wikipedia.org/wiki/Substance_P

Good question, one that I don't know the answer to. But I know an experiment that can help us find out! Spicy-hot is a complex phenomenon, but it is my understanding that a large part of it is that the capsaicinoids react with the heat receptors in your mouth to adjust their baseline temperature; making them react to normal temperatures inside your mouth as if they were physically burning hot. If tolerance is a purely mental exercise, then I would expect that I would not be able to accurately judge temperature of things in my mouth after consuming very spicy food; if I can accurately judge temperature then this would suggest that there is a biological mechanism at play.