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It sounds like what you have is a root maggot problem. There are many species of root maggots so their color may vary from one to another. However, their taste for root crops, like turnips and radishes, is the same. Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/root-eating-insects-identifying-vegetable-root-maggots-and-root-maggot-control.htm

Yes. Select young, medium-sized rutabagas. Cut off tops, wash and peel. Then cut into cubes and blanch these for about 3 minutes. Allow them to cool, drain and pack, leaving 1/2-inch headspace. Seal and freeze.

Hopefully, this article will help: https://www.gardeningknowhow.com/edible/vegetables/rutabaga/planting-rutabaga.htm
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Personally, I've never heard about removing the older leaves to promote growth. You could experiment with a few plants.

This can occur for several reasons. Most likely is that it may be getting too warm too fast for them. They start to bolt before a proper root forms.

It may also be that the soil is too hard. Or there may be a lack of phosphorous in the soil as well.

Have your soil tested and try to plant them during the coolest weather they can tolerate.

I would not recommend using borax on your garden. While it can have beneficial effects, if not used exactly in the correct amounts, it can poison the soil. It is just not worth the risks.If your swede was very bitter, this is due to stress on the plant. Make sure that your soil is well amended and that the plants are getting all the water they need during the growing season. Also, keep a close eye out for pests and disease and treat as soon as possible when you find them. This can further stress a plant.