I have a HTML block created and inserted as a ‘header banner’ yet it doesn’t appear correctly stretched side to side on desktop and isn’t appearing at all on mobile.
As I can understand, you want to make the image always stretched and don’t crop it. Unfortunately, it is not possible with our header banner. It always has a fixed height. So if you stretch your image it will be cropped because of the fixed header banner height.
You need to enable “Contain” background size for the image and then try to “play” with the image size and the header banner height. Let us know how it works and send us a link to your website.