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I doubt what you're seeing is termites. Sounds more like rhubarb curculio, which burrow in from top down and commonly affect these plants. Here is more info: http://www.entomology.cornell.edu/cals/entomology/extension/idl/upload/Rhubarb-Curculio.pdf

This is a sign that the plants have a fungus, likely black spot. Treat the plants with a fungicide and that should clear up the spots.

Although it takes about a year or two longer to grow them from seed, it is still possible. This article should help: http://www.rhubarb-central.com/rhubarb-seeds.html

However, I would recommend growing them from crowns and cutting the blooms prior to seedpod development. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm
https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm

Yes, this is true. You should stop harvesting in early summer so the plant can recover. This article can help you with your question: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm

This article should help: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm

What is it doing exactly? I cannot help without knowing its symptoms. In the meantime, here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm