Nicotine is a stimulant - not even a carcinogen, so I think that you're worrying too much. I wouldn't suggest that you try diluting and mixing in the tank while it's on the PV. I assume that you don't mix your own juice. If you keep worrying about this, I would just suggest that you buy some lower nic-content juice - maybe 12 mg - try it, and see if that satisfies your nicotine fix.
By the way, I vape 4 to 6 ml of 18 mg content per day, and I'm MUCH better off than when I was smoking. In fact, I'm feeling better all of the time.