Bart, you may have more than one thing going on, which would be typical. But, dealing with the "things" that are on the leaves and can be removed should be a first step.
As I explained before, scale and mealy bugs resist sprays by building a protective coat over themselves. There are only 4 ways to get to them that I have found. First is to use a systemic pesticide. This is NOT sprayed. You have to apply it to the soil where it enters into the plant and circulates through the plant. When the insect feeds on plant juices it takes in the systemic pesticide and is killed. Safari or Bayers Tree and Shrub are what you need to try. I recommend Safari because it does not stimulate spider mite egg laying the way the imidacloprid in Bayers does. Use it at the highest rate recommended on the label.
The second way to deal with these insects that are protected by armor is to suffocate them by covering them with horticultural oil that is not diluted. Use a paint brush or a small spray bottle to apply the oil straight out of the bottle. BUT, the oil can damage the leaves if sprayed the normal way. You have to apply the oil only to the insects and not cover the leaves with it. Works best with snow scale that live mostly on the woody stems that are not damaged by the oil.
The third way is to use a Limonene spray. This stuff is derived from the peels of citrus fruit and is a powerful cleaner and degreaser. But, at the right dose it is also an excellent insecticide. When sprayed on a mealybug you can watch the armor melt right in front of your eyes. I've only had one outbreak of mealybug and it was spreading rapidly until I used limonene to stop it cold. No mealies since. It may not work as well on scale but I can't say because I haven't had any scale other than Florida Snow Scale and I kill them with undiluted horticultural oil.
If I was in your situation I would apply Safari to the soil of all my pots and then spray the leaves with limonene (careful you get a correctly diluted dose or you could kill all the plants). Forget the other sprays you mentioned, they are worthless against adult mealies and scale. Here is the page for more info on limonene. http://www.hiddenvalleynaturearts.com/acatalog/limonene.htm
The flying dust particles you mention are just that. Insects leave waste and shed outer bodies as they mature and these dry up and stay on leaves until the leaves are disturbed. As long as the "dust" settles and does not actually fly about it's nothing to worry about.