So as far as going 6 hours on and 18 off, I wouldn't suggest that. As far as going with MH during flowering, I think that is ok to do to get some additional UV-B radiation going on.. This is important because UV-B radiation has a direct effect of increasing THC. HPS, and LED's don't offer much (nothing for LED's) in UV-B. So now for the other suggestion of keeping the lights off for 1-3 days. I wouldn't suggest going 3 days. I think 24-36hrs is where you want to be. The idea is that you are tricking your plant into thinking it's natural life cycle is ending very soon so the plant does one more ditch effort to get pollinated and produces tons of resin. This resin doesn't mean the weed will be any stronger though. It just increases bag appeal.. UV-B is where it is at kids.... This is why the strongest weed will still be produced outdoors. Can't beat the sun still. But I prefer indoor as the consistency is just better.
One more thing.... If you try and use MH bulbs the last few weeks to boost UV-B, remember that if you are using an air cooled reflector, that the UV-B basically won't reach your plant as it is absorbed by the glass of the reflector.... Just my 2 cents.... Good luck...