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It may need more sun or it may need more phosphorus. I would move it to a sunnier location and add a phosphorus rich fertilizer.

It still could be insects or frass from insects. Some pests that affect this plant, like mites, are so small, they are hard to see details of them or even see them at all. I would recommend treating the plant with a pesticide.

As long as the stems are still alive, it will come back. This article will help you with caring for it with the damage: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

As long as the stems are still alive, it will come back. This article will help you with caring for it with the damage: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

This article should setermine if the plant is still alive: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm

You don't need to deadhead them as they will self clean, but if the buds don't fall off fast enough to keep the plant from looking a little ragged, it will not hurt to remove the dead blooms.

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