Master Challenge: Auditing a Broken Modal Dialogue

Put all keyboard and focus principles into practice in a real-world debugging audit:

A QA tester files a Critical P0 Accessibility bug for your team's newsletter subscription popup:

1. When the modal opens, keyboard focus stays on the page background.
2. When tabbing inside the modal, focus leaks out into the footer links behind the dark backdrop.
3. Pressing the Escape key does nothing; users must use a mouse to find a tiny 12px 'X' button.
4. When the modal is finally closed via mouse, focus jumps to the top of the body, disorienting the user.

How do you architect the complete, robust remediation?