Can you add the slider you’re using inside a widget? If you can then you’d have to put it in the front page 1 widget location, and then edit the CSS to make the entire widget area full width. That is, unless you want the slider to be 1200px wide like the inside of the front page 1 section is set to currently.
The background image of that FP1 widget shows a background image, but that is separate from the widget (sort of) it shows up behind the widget. If that makes sense. You can remove the background image, make the inside of the widget area full width instead of 1200px, and then add the slider. That should work.
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