Hello,

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.

Best Regards.