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I would simply allow them to dry out and then replant them later. This article will tell you how to store them for the winter: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/calla-lily-winter-care.htm

Additionally, here is an article on caring for the plant that you may find helpful: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

Yes, once the leaves have turned yellow or brown, you can cut it back.

You can. It will not hurt the plant, but it will not help it either. Pruning the old flowers is done more for aesthetics.

It is either the fertilizer or the sunlight. If you use fertilizer, you may have given them too much nitrogen, which encourages leaves and discourages flowers. If you suspect this may be the case, add phosphorus. Here is more info on phosphorus: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

A lack of sunlight can happen if a nearby plant has grown to shade the area. Check the area and see if the calla lilies are getting enough sun.

I'm assuming these are grown as houseplants. That being said, browning of leaf tips is generally normal and will not harm either plant. This is oftentimes the result of fluoride found in water, which causes salt buildup in the soil. It usually helps to periodically leach plants by giving them a thorough watering to flush out excess salts. Be sure to allow the water to drain out and repeat as needed. It may also help to use distilled water or rainwater when irrigating plants.

This article will help you: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm