What is the name for the 'sweetness' of cooling roast pork?
I don't think there is a word specific to the "sweetness of cooling roast pork" in the English language. This is simply an "aroma", and more specifically, the "aroma of roasted pork". (Or, in my house, "the smell of porky goodness") It is produced by caramelization of the sugars and the Maillard reaction that occurs during roasting.  These are the same mechanisms at work when you caramelize onions, as you point out.