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Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/herbs/mint/invasive-mint-how-to-kill-mint-plants.htm

First, you'll want to make sure that all the spearmint has been removed, as these plants are somewhat invasive. Since you have had problems in the past with squash, you may want to get the soil tested to find out what nutrients may be missing in the soil, as well as the pH level. Once you have done this, you can correct any issues.

Sprigs and stems are essentially the same thing. Typically you would harvest it like you were giving the plant a haircut. Simply cut off what you think you need.

It sounds like the plant may be dying after harvest due to disease getting in the open wounds. Trim only with very clean scissors and shears. You can wipe them off with rubbing alcohol even to kill anything that may be on the blade that could hurt the plant. Then, after you harvest, provide a little more water to the plant so that it is less stressed and can fight off disease better.

Bacillus Thuringiensis (Bt) spray can quickly eradicate these leaf and stem-eating worms. It is a safe, biological insecticide. In fact, Bt a bacterium which is toxic to many moth and butterfly larvae (caterpillars) and will not harm the plants or other beneficial creatures, including us. In addition to using Bt in the garden for worms and caterpillar pests, you can try neem oil. Both are safe to use. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

spearmint will get up to 4 feet tall and can get wide. it may need to go elsware.

Yes, you can do this and they would be a mix. And yes, it would only affect the seeds. Certainly worth it to try.

Basil will cross with other basils and oregano with oregano and sage with sages but not with each other.