Will this cause brake light interference or cutout issues?

No. This is a rear lip diffuser mounted to the bumper, not a trunk spoiler. It sits below the brake light assembly and does not obstruct lighting elements.

Does it stay stable at highway speed?

Yes. The one-piece FRP construction is rigid and designed to handle sustained wind loads. Proper mounting to the rear bumper ensures it won't rattle or shift during normal driving.

Can I use adhesive to mount this, or do I need to drill?

This diffuser is designed for bolt-on installation to existing rear bumper mounting points. Adhesive alone is not recommended for sustained highway use—bolted mounting provides the stability this piece requires.