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Hardy hibiscus does not need any winter protection in your area and should not need any special protection.

This article will help with pruning:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm

This article has recipes for homemade sprays:
https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm

I like neem oil myself. You have to buy it, but it is organic, safe for people, pets and good bugs and very effective. Here is more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

This sounds like lichen, which is actually a symptom rather than a cause of the problems on the trees. Lichen often like trees or branches that are dying or dead, which leads many people to think that the lichen is to blame for the problem.

If your garden is often waterlogged, you may have a fungus in the roots of the trees that are killing them. Are you seeing any other signs of problems in the trees? What do the leaves look like when the branches are dying?

If the flower is older, it may just be fading naturally and that will cause it to brown. If it is fading sooner than you think it should, it is either lacking enough light or is over or under watered.

Move it to a brighter window and water the soil only when it is dry to the touch on the very top of the soil.

Perhaps this article will help: https://www.gardeningknowhow.com/edible/fruits/peach/brown-rot-fungus.htm

They only produce one bloom a year, but you can help them bloom well for next year. Here is an article that will help you: https://www.gardeningknowhow.com/ornamental/bulbs/hyacinth/planting-and-care-of-hyacinths.htm