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No need to cover hosta, I am in zone 5, never a problem.

I suspect something is chewing on the roots of the plants. Hostas are hardy plants, so it will not hurt one to dig it up and take a look at the roots and see if you see damage. The pests would be voles, mice or even squirrels or chipmunks. If you see damage to teh roots or find burrows, you will need to take steps to get rid of these pests.

You won't be able to propagate hosta this way but you can divide the plants to get more and now is a great time to do it. Simply dig up the clumps you want to divide (usually the largest) and split them with a spade shovel, getting anywhere from 2-4 (or more) smaller clumps, depending on the overall size. Make sure each new clump has some roots with it and replant elsewhere. These articles should help you as well: https://www.gardeningknowhow.com/garden-how-to/propagation/propgen/dividing-plants.htm, https://www.gardeningknowhow.com/ornamental/foliage/hosta/transplanting-hostas.htm

This is typically caused by a lack of water, but could be caused by a fungus as well.

I would recommend that you make sure the area is getting sufficient water, keeping in mind that trees in the area may have been gradually taking up more and more water over the years so what may have been enough water a few years ago may now be too little.

And, as a precaution, treat the plants with a general fungicide.

It could be slugs, which feed at night. This article will help: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm It may also be rodents. This article will help you with that: https://www.gardeningknowhow.com/plant-problems/pests/animals/garden-mouse-control.htm

Yes, it may have been voles or some other rodent. This article should help: https://www.gardeningknowhow.com/plant-problems/pests/animals/vole-control.htm