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Yes, this is fine. The tops can be harvested at any time. Just make sure to leave some for the root to grow.

This brings to mind two possible issues. One would be powdery mildew, and the other would be aphids. Any pictures would help me to identify the issue. Luckily they can have the same treatment.

Neem oil is a good treatment for a broad range of issues, and can be purchased online, or in gardening centers. Just have care and do not spray in bright direct light. Wait for a cloudy day, or early morning.

Here is a collection of articles for more ways on controlling mildew, and other pests: https://www.gardeningknowhow.com/special/organic/

Si, puede fertilizar en este momento- puede anadir el fertilizante al lado de la planta. El boro es importante para col de bruselas:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/boron-on-plants.htm
-Alisma

It could be the wind, or it could be that they are being stepped on by a person or an animal. If it is a small animal it doesn't appear to be eating the plant. This could mean that it is a bird, or something looking for insects, and accidentally snapping off leaves. There is little you can do for this case, and the plant will often outgrow the damage that this will do rather quickly.

Unfortunately there is not a lot that can be done here. If you can, temporarily, net the top, then this might help some.

Of all the factors in growing conditions and maintenance that contribute to plant health and productivity, soil and water management are most important.

You mention mulch with grass clippings, but if that all you did to improve soil fertility then maybe its not enough to acheive the nutrient availability that the plants need to flower and produce.

Next season incorporate some good quality compost into the soil before planting. Fertilize with a complete organic fertilizer like this:
https://www.domyown.com/dr-earth-home-grown-tomato-vegetable-herb-fertilizer-p-9493.html?sub_id=9492

Grass clippings alone, as mulch on the soil surface often form a matted layer that does not allow good water infiltration. Best to mix with a courser material, or compost the grass and then use the compost as mulch.

I would let the plant mature just a little before adding other flowers. You wouldn't want them to outcompete the sprouts. Once it is established, then planting some things like nasturtiums and marigolds can actually help out by keeping pests away from your plants.

This article will give you some more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/repelling-bad-bugs-with-plants.htm