Greetings and Regards
1- I set the width of the site to 1280 pixels in the template settings, but the width of the site is broken on the shopping cart and checkout pages.
2- When I use the existing template headers or a new header, the whole header template crashes.
01. Navigate to Pages > Edit cart and checkout page > please make sure that shortcode is added properly for cart and checkout pages.
Like this: https://ibb.co/dpsP9Xs https://ibb.co/KNXNNXJ
02. I have not seen any issue about the header on your site. Please, describe to us your problems in a bit more detail. Attach some screenshots and provide us with a list of steps on how to reproduce them on your website.