Will this scrape on steep inclines or driveways?

The diffuser adds minimal extension below the bumper line. Check your vehicle's current rear bumper clearance at your typical approach angle — if the OEM bumper clears, this diffuser will follow the same path. Steeper driveways may require adjusted entry speed, same as stock.

Does this actually reduce rear lift, or is it cosmetic?

This is a functional diffuser. The geometry works with airflow to manage rear-end stability at highway speeds. You'll feel the benefit most on sustained high-speed driving; daily street driving gains are more subtle. It's not a wing, so don't expect dramatic downforce numbers — the value is in balanced, effective aero without the visual aggression of a large wing.

Should I apply clear coat over the carbon fiber?

Carbon fiber benefits from UV-protective clear coat to maintain its appearance long-term. Sand and prep the surface before applying your chosen finish. A quality automotive clear coat will seal the weave and resist fading from sun exposure.