What is the name for the 'sweetness' of cooling roast pork?

I think you are confusing a lot of things here. First, you apply a cooking technique to the food. Second, the cooking technique causes a lot of physical processes to happen within the food. Third, the cooked food causes sensory perceptions in your head. I once explained the difference between cooking techniques and physical processes in this answer, so won't repeat it.

That being said, I don't think a word for your situation exists, on any of the three levels. The word for the cooking technique you used is simply "slow roasting". There were many different physical processes going on in the meat, including protein denaturation, collagen converting to gelatine, caramelization of sugars, maillard reaction between proteins and sugars, and so on. And then you ended up with a smell of slow roasted pork, which is a perception.

None of these words describe your situation unambiguously. Had you used a different recipe for roasting the pork, it would still have been "slow roasting", but it would have smelled differently. None of the physical processes is by itself responsible for the smell. It's the combination of them, and with the unique parameters they happened to have, which produced exactly this smell. And the smell itself doesn't have a name either. The olfactory sense of humans is incredibly complex. We can sense 5000 different substances, some of us can sense substances others can't, and they all produce a different sensation in different combinations and concentrations. We don't have the words for every possible smell in the world, neither do we have the capacity to remember or distinguish them all. So, language has no better word for "the smell of slow roasted pork with apple juice glaze" than this descriptive phrase. Whoever has smelled it, might remember it. Whoever hasn't, probably won't be able to imagine it even from the description.

You are welcome to describe the smell any way you want, and you are correct to tell the people that your technique was "roasting". The fact that they won't be able to perfectly picture the exact sensation of your food is just an everyday thing in human communication.

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.