Single Cat5e for Computer Network & Telephone?

That's not creating energy. It's merely conducting energy from another source, such as a radio station. Then again, if that was a problem, it wouldn't likely require the presence of a phone line in the CAT5 cable to make it one.

Yes, as I mentioned earlier, corroded connections can generate harmonics, but they still cannot create energy. It can, through the generation of harmonics, affect the frequency distribution of the energy, but not create more than there was to begin with. And again, the harmonics will have reduced amplitude with harmonic order. It's been a long time since I've looked at the harmonic distribution of a perfect half wave rectifier (the best you can get with a perfect diode), so I don't recall the exact details. However, here's some info about harmonic distribution, in both full and half wave rectifiers. Don't forget, corrosion is not likely to form a "perfect" rectifier, so the amount of harmonic energy will be even less.

I've never claimed a perfect world, but some people seem to be making some extraordinary claims about potential problems, without examining the real world facts, that must follow the laws of physics. Incidentally, a phone line that generates significant levels of harmonics, does so by creating significant levels of distortion in the desired signal. Such a line would not go unnoticed for long.