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I am sorry to hear about your plants. This article may help you with getting them to recover: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

Hibiscus flowers are not thought to cause allergy problems, but if the plant has a fungus or a bug infestation, this could trigger allergy problem. I would check the plant over carefully, particularly where the stems meet. Look for cottony, scale like or powdery residue. This will be small pests and can be treated with any standard insecticide. If the leaves have a powdery covering or have brown spots, this is a fungus and the plant can be treated with a fungicide.

This article will help you with starting plants from cuttings:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-propagation.htm

This article will help you with grafting:

https://www.gardeningknowhow.com/ornamental/trees/tgen/grafting-trees-what-is-tree-grafting.htm

Yes, it should be ok to move now in you area. Make sure to give it plenty of water the first week or two after you move it.

This is pretty common for hibiscus and is normal. The lack of sunlight causes them to head into dormancy and drop leaves. You can try to stop it by adding supplemental light from a lamp, or just let it go semi-dormant until spring, when it will perk up again on its own.

Semi-dormant simply means to let it stay in a state where it loses its leaves somewhat, but a few stay on. It does not look very pretty, but it is difficult to keep hibiscus from doing this anyway. I would cut back the water some during this time, only watering when the soil is dry to the touch.

Try using neem oil. I have found that nothing works better than this for scale and aphids. Here is more information on it: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm