I don't think it will
profit you at all to worry.
This is another situation where you likely have collected all the debris you can without seiving out your soil. I am pretty sure the worst part of the asphalt shingle is the asphalt; the parts that are most likely to rub off are the tiny rock particulates that coat the surface of the shingle to protect the asphalt and underlay from UV exposure.
In a similar situation (our old garage roof was blown to pieces into our back yard at my parents' house), I piled on the woodchips, made
mushroom slurries and applied finished
compost and compost extracts. Any heavy metal pollution will be sequestered in the fruiting bodies of the mushrooms, and other contaminants will be broken down and redistributed by the mycelial network to places they are needed.
The tiny particulates aren't leaving your property unless you go to extreme (and probably very harmful) measures, so I think the best
course forward would be to
boost the vitality of the soil through the methods I mentioned above, and let the soil life break everything down. If you are concerned about specific contaminants or spots, perhaps consider growing sequestration crops that you then use as mulch somewhere you don't grow food, like a
wood lot.
But don't worry. That won't do anything.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein