Will this interfere with my trunk seal?

No. The rear lip mounts to the bumper structure below the trunk line, so there's no contact with the seal. Measure the clearance between your bumper and trunk edge before drilling to confirm your specific vehicle's fitment.

Do I need to drill into my bumper, or can I use adhesive?

Bolted mounting through the bumper is the most stable approach for a rear lip. Adhesive alone may not handle highway speeds or rough road feedback. Confirm your bumper's material and existing hole locations before committing to either method.

How should I prep the FRP before painting?

Sand the surface with appropriate grit to scuff the finish, then wipe clean and mask around the mounting area. Follow your paint manufacturer's guidelines for primer and topcoat adhesion on plastic substrates.