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You should store hibiscus where it remains over 50 degrees for the best results, however, they can survive light frosts on occasion. When in doubt, bring them inside. This article wil help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/wintering-hibiscus-indoors-winter-care-for-hibiscus.htm

You can either start with dried flowers, or fresh. I prefer dry, though. Steep them in a tea ball or mesh as you would regular tea bags.

The dead trunk can be left in place, as long as it doesn't appear diseased or rotten. You can prune off the dead branches and foliage from above the braid. Since the rest of the tree looks healthy, there is probably no disease concern.

This depends... There are hardy hibiscus, and tropical. If you have a hardy hibiscus, they pretty much survive everywhere. The tropical will not unless you bring them in. I would need to know more information.

It would be a good idea to prune off any dead or dying growth, and deadhead any spent flowers. Here is an article with more information: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/

I can only recommend checking your local nurseries, and potentially ordering online.

In the meantime, here is an article on their care: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/outdoor-hibiscus-care.htm