There are probably humans that are immune. Dogs, for instance, are known to be mostly immune, despite being susceptible to infection.
But the most likely hypothesis for humans is that the volume of the exposure matters. I.e., it's more dangerous to handle an almost-dead patient than to be in the same apartment with a new patient in the process of getting sick enough to be hospitalized.
True, but it only takes a couple of viruses to get infected. The biggest risk is getting exposed to a late stage patient as you said, but before that point it will depend on cleanliness, luck, etc. In the case of Youngor Jallah, she did a lot of disinfecting. Perhaps Duncan washed his hands also after he used the bathroom.