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The plant sounds like it may have a bit of powdery mildew and possibly flea beetles. Here is information both these things:
https://www.gardeningknowhow.com/plant-problems/pests/insects/what-are-flea-beetles.htm
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm

We recommend treating the plant with neem oil. It is safe for humans and pets and treats both problems your gardenia possibly has. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

You will not be able to make the whole plant regrow its root. They just would not have the energy for it. But, you can try taking the plant and using it for cuttings to make new gardenia plants for your yard. This article will help:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/propagating-gardenias.htm

Times like this can be difficult for gardenias. For the heat, I would make extra sure that the plant is watered well. Check it twice a day.

When it gets too cold, cover it at night with a sheet or light blanket and make sure to take it off first thing in the morning. The covering will help hold in heat and keep away any possible frost that may occur. It is not foolproof, but it will help.

In the meantime, as long as the stems and branches are still alive, the plant can regrow the leaves. Cut back fertilizing until the leaves return, but make sure that every thing else in terms of water and light are provided.

I would definitely cover them up for the night. This article can help you with this: https://www.gardeningknowhow.com/plant-problems/environmental/protect-plants-in-freeze.htm

This article will help you with the mealy bugs:
https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm

We have recently started using neem oil on out plants here. It works very well with pests like this. You can learn more here:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

You can prune now, but you will reduce the number of blooms you get on the plant in the coming year. It is recommended that you wait until after it blooms to prune so that you do not lose the flowers.

You can take cuttings from the pruned branches. This article will help with that:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/propagating-gardenias.htm