Will this interfere with the trunk seal or weather stripping?

The Broman design is engineered to sit flush with the factory bumper line and should not contact the trunk seal. Verify clearance by dry-fitting the outer pieces before final adhesive or fastener installation, especially if your bumper has any previous damage or has been repaired.

Does this diffuser cover or block the brake light area?

This rear lip sits below the brake light housings and does not obstruct them. The three-piece configuration allows you to mount each section independently, so you have full control over positioning if your bumper sits higher or lower than stock.

Should I glue or bolt this down?

Both methods work. Adhesive offers a clean appearance and requires surface prep before bonding. Fastener mounting provides easier future removal if you decide to swap styles. Assess your bumper's material and thickness, then choose the method that best suits your installation goals and the condition of your bumper surface.