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You can try putting the plants into dormancy. Place the plants in a cool area, such as a garage, basement or, if there is a crawlspace, under the house. The plants will appear to die back but still water them about once a month. When the weather warms up, take tham out, place them somewhere warm and resume regular watering. They should regrow quickly.

No, plants indoors are not harmful. Quite the opposite, actually. It has been shown that keeping plants, especially palms and ferns, can greatly improve the air quality in the home. The plants not only increase the oxygen levels inside, but they remove toxins from the air as well.

This article should help: https://www.gardeningknowhow.com/ornamental/foliage/ferns/transplanting-ferns.htm

Is this fern in a pot? It may need to be repotted. This article will help you with that: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm

It could also be a pest problem or a fungus. I would recommend treating the plant with neem oil. Neem oil will treat both fungus and pests. Here is more information on it: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

This article has suggestions that should help: https://www.gardeningknowhow.com/plant-problems/pests/animals/getting-rid-of-snakes.htm

I don't think it is a sign of anything serious, but it may be a sign that the plant needs to be divided. I would try dividing it. This article will explain how to divide a plant:
https://www.gardeningknowhow.com/garden-how-to/propagation/propgen/dividing-plants.htm