First rinse all of that other stuff off . A little household ammonia in a spray bottle will kill them. Repeat in a week. Your plants will grow new leaves!

The plant has been rinsed many times. Do you recommend that I dilute the ammonia? If so, at what rate?

I have tried diluted ammonia too actually and the leaves burned. What did I do wrong?

Thanks...

Ok- good. I add a tsp per gallon. That should do it! Maybe you added too much? You can always strip off all the leaves and they will grow out again.

Neem oil diluted according to instructions and sprayed on the plants, especially the undersides, help my plants. It needs to be done over and over again when there's an infestation. Right now I'm remembering about 1x/week. They're outside by the way.

I have used horticultural oil (brands such as Sun Spray, Ultra Pure, Ultra Fine) for years with excellent results.
Follow the label directions, typical rate is 1 ounce of oil to 1 gallon water.
Spray thoroughly, undersides of leaves, stems, really soak it. The oil works by suffocation, it is not toxic, the mites can not breathe, and that is that. Very safe to humans, pets, children. Inexpensive, very effective, no resistance and no odor.
Couple of notes. Do not use if the temperature is expected to exceed 85 degrees. Also it can damage the blooms. Apply it twice, about 14 days apart, this will get anything that was missed on the first spray, and you should be clean.

On the Neem oil, if you have potted plants inside, take them out to spray. It stinks!

Do hope you can clear up the problem
California I believe is the very best place to grow Brugmansia Sacramento should be just ideal
I wonder why you got the spider mite in the first placeI do not use sprays etc for plants in Canada or at my home in Florida. But I do try to come up with a reason for any problem.

It seems to me most times there is something that can be changed that is the cause of, in this instance spider mite.
Hope you can resolve the problem And yes I agree take all the leaves off for a try of it. No time at all, new leaves will be back on the plant.