There's no magic fix for hyperpigmentation unfortunately. The only thing you can do is give it time. Especially on Accutane.

I have read that azelaic acid (skinoren) can be good for lightening skin, but it can also cause discolouration of the skin.

Have you ever looked at a product called 'genius nightly exfoliating pads' from Arbonne that's what I use (personally on my acne brown spots) and it's fading them all slowly but surely. I could send you a whole file on it if you would like