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If you notice any signs of worms or insects, which sometimes appears in cabbage, soak the head in salt water or vinegar water for 15-20 minutes first

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

You can remove the plants and compost them.

It is possible that the butterflies are laying eggs, and that they could potentially harm your plants. I have a recipe that I have developed to cure insect issues, fungal issues, and to correct any slight deficiencies, and pH issues all in one go. My recipe is as follows:

First, you will want to prime the soil with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.

Yes, I would wash the cabbage and then you are safe to eat it.
Here is a link with information on Cabbageworm.

https://www.gardeningknowhow.com/edible/vegetables/cabbage/protecting-your-cabbages-from-cabbage-maggot-cabbageworm-and-cabbage-moth.htm

This is part of the cabbage plant, yes. What most people consider to be "cabbage" would be the tightly formed center mass of leaves that form much later. Your plant is still young, and has not had time to form fully yet. If has been too long, then you might consider if the plant is getting enough light. They are full sun, and will prefer to be in bright sunlight all day. Here is an article that will help: https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage.htm

Can you include pictures? I'm likely to say insect, or maybe even infection. Is there any other symptom such as coloration, or anything else? There is also a possibility of birds, if the top is exposed.