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You need a minimum of 8inches for proper root growth.

Though you can consume the Kale after bolting, it may be more bitter and not as palatable.

https://www.gardeningknowhow.com/edible/vegetables/kale/harvesting-kale.htm

Yellowing could be caused by watering issues, but due to the fact that you are having this issue with different plants and you may have had this issue in past years, I would say a fungus issue.
The links below will help you.

https://www.gardeningknowhow.com/edible/vegetables/kale/kale-plant-protection.htm

https://www.gardeningknowhow.com/edible/vegetables/kale/growing-kale.htm
https://www.gardeningknowhow.com/edible/vegetables/greens/growing-collard-greens.htm

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Treat the plant with Neem Oil.
Neem Oil is organic and safe for people, pets and bees.
Yes, you can trim the damaged leaves from the plant. Try increasing the sunlight available to your plants.

https://www.gardeningknowhow.com/edible/vegetables/kale/growing-kale-in-pots.htm
https://www.gardeningknowhow.com/edible/vegetables/kale/kale-plant-protection.htm

I would try marigold, or nasturtium! These two keep bugs away pretty will. You might even try citronella geraniums, as these are known to repel insect pests, as well. Here is a link that has even more methods: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/repelling-bad-bugs-with-plants.htm

You can trim the dying growth, but I would not feed until spring. This fertilizer would sit in the soil, and since it wouldn't be taken up through the winter it would sit in the soil. After awhile it will lock itself up in salt form and be unavailable to the plant. Although they can take the cold weather, you should either move it or cover it on the worst nights. Otherwise, It will recover just fine.