Do I need to remove my factory grille to install this?

Yes. You'll need to unbolt the factory insert from behind the grille shell, then bolt this overlay into those same mounting points. The factory grille frame stays in place.

Will this overlay sit flush with my grille opening, or does it stick out?

Mounting method not specified in product data—test-fit the overlay against your grille opening before final installation to confirm alignment and any slight gaps or protrusion.

Can I polish the chrome if it gets scratched, or do I need to replace it?

Light scratches in chrome can be buffed out with metal polish and a soft cloth. Deep gouges or pitting will require replacement—chrome plating cannot be field-repaired.