KM Thank you for the post, that is pretty amazing. I see on the first image the freq is almost 60Khz, what soundcard are you using?
I must say for myself, I tried measuring iphone 6s, that gives me one of the worst eyestrains and could not detect any PWM. What I detected was weird flickering lines when hitting the lowest brightness levels. I detected the same ones on my iphone 4s though, that is the most comfy device I can use for hours on end. I blame my non-professional gear and wanna test again with some proper gear, as I am absolutely convinced even the newest iphones use PWM - my symptoms are the same with them.
EDIT: Try playing with the soundcard dials if it has some. I noticed sometimes the amplitude was very low, and then realized I have a very small gain on the card.