As you said, the container is set to 1100px – the width of the browser/viewport is much larger than that.
If you set your header to contained, then it would be set to 1100px, but it’s set to full width so it spans the entire width of your screen.
Stretching the image is possible, but you’ll lose quality. What you’re done is the best method by far.
As for the disable elements not working, that’s strange. Do the checkboxes stay checked when you save?