Hi. I can't speak for this out of personal experience -- but here's a thought. Boaters have a similar problem -- the plastic windows on their canvas boat covers yellow with age, and there are some good products (I've seen their fantastic results, but on canvas windows, not Volvo headlights) that you can buy at boating shops. Why not try a bottle (see what your local boating shop stocks, and try it) -- the problem seems similar, and if you can see out of the canvas windows, maybe it will be good enough for a headlight.
Just a thought. Good luck.