I've got a lot of leaves turning yellow on my plants too which includes plumerias. I think the hot weather has a lot to do what it. The leaves that grew during that nice cool wet spring can't be supported in the heat and so they drop. I don't grow hibiscus but the heat may very well be responsible for the bud drop also. The black and white debri sounds like it could be spider mites or white flies. Spraying the undersides of the leaves with water every three days for 9 days in a row is recommended for spider mites on roses and neem oil works well for white flies. I've seen that little white thingy you've pictures on leaves and figured it's some sort of very small cocoon.

Hopefully someone else will chime in with some more ideas. I didn't want your questions to go unanswered so this will take it back up to the top.

Looks like mealy bugs to me. Horrible pests and nearly impossible to get rid of. It will take consistent spraying on whatever intervals your insecticide states. Be prepared to watch for them and spray for them for quite some time as it is always, always nearly impossible to kill each one present whenever you spray. Also, check under the rim of your pots. They will even burrow down into the soil. Did I say now horrible they are (disgusted voice tone fully intended). Good luck.