Will this interfere with the stock bumper or lower valance?

This lip is designed to bolt to the existing front bumper mounting points with standard clearance. Before installation, verify that your vehicle's OEM bumper attachment holes align with the lip holes—minor drilling may be required if your bumper has been previously modified or if aftermarket components have been installed.

Should I apply clear coat, and what prep does the carbon need?

Carbon fiber benefits from UV-protective clear coat to preserve the weave finish and resist fading. Sand the surface lightly and degrease before application. Use your preferred clear coat product and follow its cure time before driving. Uncoated carbon will yellow over time if exposed to direct sunlight without protection.

How stable is carbon at highway speeds, and will wind load affect fitment?

Carbon fiber is rigid when properly mounted to factory attachment points and handles highway speeds without flex or rattling. Ensure all fasteners are tight before first drive. Check torque values against your vehicle's service manual, and re-verify fasteners after your first 50 miles to catch any settling.