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There are some heirloom varieties of rhubarb, such as Victoria, that have green stalks. They are perfectly fine to eat.

Well, they do tend to require a dormancy below 40F to be able to produce well. If you want to simulate that you can put it in a lightly moist soil in a ziplock bag. Keep this on the top shelf of the refrigerator. This will give it the conditions, for at least a month, to produce again. This article will give you more information on the plant: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm

Yes, the tender emerging shoots are the most sensitive to cold and frost.
An old pillow case, milk jug or bucket can be used to cover the plants. Make sure you remove the covering as soon as the temps rise in the morning.

This can, actually, be a bit of a challenge in warmer areas. This article will give you more information: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/rhubarb-in-hot-climates.htm

There are many things that you can put into the soil. I recommend a few things. Dolomitic lime, compost, worm castings, iron sulfate, and, if you would like, some perlite to assist with drainage.

It could be slugs. The best way to tell is to go out at night with a flashlight and try to find the culprit. Caterpillars also make good size holes but they are visible in the day. Take a close look as some have amazingly good camouflage. If it turns out to be slugs, here is an article on controlling them. https://extension.psu.edu/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants. The article mentions using egg shells to deter slugs but studies have shown that this doesn't work well. Baits like Sluggo and Sluggo Plus are safe for birds and pets. Also consider covering your rhubarb with a row-cover material during the night. You'll need to tuck it in right next to the base of the plant.