Will this trim sit flush with my existing bumper cover, or does it need gaps sealed?

This mounts at factory attachment points and should align with your OEM bumper cover. Test-fit before final fastening to confirm gaps are within acceptable range for your specific vehicle's bumper condition.

Does the chrome finish need any prep, or can I install it straight out of the box?

The chrome is ready to install as-is. If your bumper has been repainted or repaired, clean the mounting surface thoroughly to ensure solid contact at attachment points.

What happens if my lower bumper valance is cracked or damaged — will this trim cover those imperfections?

This trim runs across the upper bumper perimeter only. It won't hide damage to the lower valance or lower sections of the bumper itself — those would need separate repair or replacement.