Super interesting post, thx for all the information.
When I tried to play with mediaDevices I used to call it like following, but I don’t remember if I tested it on iOS < 11.3
const getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia;
could that help?
also I remember that my problem was the result, “back then” the stream had to be handled differently in Chrome and in Safari 
Did you try with Capacitor? I tried the camera of Capacitor in a PWA on Safari and Chrome on my mac, in a bundled app on iOS and Android (all good) but I didn’t tried it in a PWA in Safari on an iPhone