Thank you for very detailed report!
These are two separate issues:
The first one - the WebGL canvas not adapting its size to the screen width seems to be a bug in some iOS & web browser combinations. We cannot reproduce it on our iOS devices, and we have not yet found a workaround for it. Does changing the orientation of the phone from portrait to landscape and back help?
Yhe second one: you are using an iframe inside an HTML label. What width did you set for an iframe? If it’s a constant it prevents the label from adapting to the screen width. To make the iframe width adaptive you could try using CSS width rules using the visual width units, eg:
width: 80vw or
width: calc(100vw - 80px) (these are just examples, you would have to experiment a bit to find values for your use case).