I am reading about plaque and I can't find any evidence that it migrates from the teeth to the lips and builds up there so quickly, but of course it's a possibility.
Now another hypothesis is that the white line of gunk is dead skin build-up that has been super-soaked by your saliva. Following this idea, if your skin isn't building up as much (keratinizing) and separating from the lip (exfoliating) every couple days then you wouldn't get the white line so badly. I actually was getting the white line for a while and am not anymore, and my peeling cycle is at about a week now.
That said, I think it's pretty clear that oral hygiene is a double-edged sword where my condition has worsened (and perhaps began) following visits to the dentist, but people who don't see dentists risk infection or secondary infection. Also, some people may have allergic reactions to their toothpaste or chemicals/metals in tap water. I am using Auromere at the present.
I still notice myself sucking on the inside of my mouth when I wake up or am stressed -- I have done this for as long as I can remember having the condition -- I am trying to stop this habit consciously and hopefully unconsciously as well over time. I have observed the skin in the middle and on the sides of the lower lip separate on their own, so I carefully cut the skin off, but the rest has to be removed more forcibly. This could be explained by what parts of the lips are the most in motion.