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I actually recommend wettable sulfur dust. A VERY SMALL amount of it. It is very difficult to treat African Violets once they get an infection, so most of the treatment will have to be preventative.

This article will give you more information on growing these: https://www.gardeningknowhow.com/houseplants/african-violet/african-violet-care.htm

This is definitely a fungal infection. You can dilute peroxide with an equal part water whn you water next, and this will help get rid of the problem. It is a chance that the soil is already suffering from a bad infection that might not be able to be repaired if it is already showing symptoms on the plant, then it can be hard to get rid of at this stage, but peroxide will be your best bet. Here is an article to refresh you on their care: https://www.gardeningknowhow.com/houseplants/african-violet/african-violet-care.htm

This is completely normal as they age... These actually look really good. If you can get them to flower, then you can be sure that you are giving them the correct care. You can just take these leaves off once they are dead.

This article will refresh you on the care of this plant: https://www.gardeningknowhow.com/houseplants/african-violet/african-violet-care.htm

I will need a photo to be able to see the damage. Most of the time it has something to do with the watering amount or sunlight amount. Could you please describe how you water and how much light that it receives, the temperatures in its area, and the amount of fertilizers that it receives?

Unfortunately, this is something that happens to the plant with age. There is no way to stop it. It is best to let it continue on, or start over with a new one until it does the same.

You will need to wait and see. If they were frozen and the roots were frozen; they will freeze more easily if the soil was dry, they will not recover.