I have to say that I am slightly bemused by all this. If the dust spots are not showing up under normal editing, then why worry about removing them?
If I use a heavy grad in the sky, then it tends to show up dust. So I remove the spots, because they will show up in the finished print. But if I can see no spots, I leave well alone. I know that if I use a mask it will show up even more spots, but if they do not print under my final edit then why worry?