Hello,
1- Regarding the Checkout page Form fields, the thing is under the Custom Layout you can only move the elements as an whole related to Shipping details and Billing details: https://snipboard.io/jw3CAP.jpg
If you want to edit them then you would need to concern the WooCommerce Support in this regard because the Checkout page comes from WooCommerce by default and editing the fields on the Pages requires Customization and this is beyond our limitations and support policy scope. Hope you can understand our limitations.
2- Regarding disabling the Footer from the Checkout page, you need to edit the Checkout page with Gutenberg editor on your Site > there under the Page Settings options > click on Footer sub-tab > and under that enable the toggle button of Disable Footer: https://snipboard.io/RpxYhy.jpg
Regarding selecting specific Footer, right now there isn’t a separate option available unfortunately.
Regarding the Header, you can’t disable it through options. A Custom CSS would be required for that.
But you can select a different Header for the Checkout page from the same Gutenberg edit > Header sub-tab > there you can select the Header from the dropdown: https://snipboard.io/nb4Rj1.jpg
Best Regards