Since it is putting on new growth, I would leave it be for now and let it recover. Make sure to water daily since it is in a pot. I have a large mophead in a pot and I give it 1/2 gallon of water every day. Because it is in a container, it will need more watering than it would in the ground. The newly planted hydrangeas I have in the ground get watered every 3-5 days. I agree with Ian about the sun scorch---and wanted to add that a water stressed plant is more suceptable to insects (I learned it first hand when my limelights (who I neglected on vacation) got spider mites. I treated them, and they are doing what yours are doing... recovering, putting on new growth, but the old leaves look awful.

An infestation of whitefly strikes complete horror when I see it (second only to thrips) as they are vectors of disease and severe infestations cause irregular ripening of tomato fruit. I have turned over every leaf of many many plants just to rub out early stages of whitefly. They are very difficult to control once established. But I am not quite sure if you mean you only have 'dead' ones --there are many beneficials that do help reduce their populations. If you tap the plants do they flutter out. Turn the leaves over , some stages are stationary but you should see movement of the adult whiteflies. If you do see live whitefly I think I would go with an insecticidal oil or soap and oil spray as they build up resistance to chemicals very rapidly. Or I might try something like spinosad which I have used on thrips. Scroll down to the very bottom of the link to see recommendation to the homeowner. I would take great care to spray under the leaves. Yes the damage you describe could be from whitefly I think but at least you are not describing symptoms of tomato yellow leaf curl virus (TYLC). I have seen a few plants with TYLC symptoms and I reluctantly just put a garbage bag over the whole plant and pulled it out. Your whiteflies may not be carriers of TYLC. I love experiments and would be interested in your MG vs fish emulsion one. Hope you can get a handle on your whitefly problem. Here is a link that might be useful: Info. on whitefly

Those little bumps do look to be scale. I've been lucky enough to not have to deal with scale, but I'm fairly certain you treat them like meallybugs. First remove the ones you can see by picking them off. Then gently scrub with a soft toothbrush and a solution of rubbing alcohol, a few drops of liquid dish soap, and dilute with a little water. You will have to do this over and over till they are gone. Check once a week. Each time there should be less and less. Once you are for sure they are gone repot into new mix and clean the container really well:). Still keep your eye on the plant. All it takes is a couple that you missed. Keep this plant away from all other plants:)! While doing this treatment keep out of direct sun. Goodluck;). Here's a YouTube video:) that you may find helpful:).

...and tell us how you’re caring for it tj