UV-C radiation has been used as a mutagenic agent in plants
UV-C is not present at the earth surface. And ya. Its bad stuff. Pulling that out does not make your argument. It is important not to conflate the properties of the various wavelengths in the discussion.
My point is that human physiology is not even a good proxy for what happens in photosynthetic organisms. And then there are other factors in plant response from ancient environments where plants have adapted to conditions where the ozone layer had been destroyed, where carbon dioxide levels rose to levels that humans cannot tolerate (~4000ppm) and average temperatures were some 10 degrees centigrade higher than what they are now. These genetic responses are exploited by greenhouse growers to maximize production.
I am interested in your thoughts however I'm not really finding any evidence in what you are suggesting for proof.
I think we can agree that plants pretty much like the spectrum provided by the sun at the earth surface. That doesn't include UV-C unless there is no atmosphere. It does include some level of UV-B and a much larger component of UV-A. We get broadband radiation from the sun and our atmosphere filters out the shorter wavelengths significantly in the case of UV-B and completely in the case of UV-C. All of the studies about plant response with UV-C was stimulated by the ozone-hole crisis a couple of decades ago where everyone in agriculture went "Oh Shit" when they realized our food sources were in imminent danger of destruction on a global scale. The science was done so the politics could get on with solving the problem - Chlorofluorocarbons. Chlorine is a nasty element when you get to know her. Reactive as fuck.