1) I have a 911 (1974 911 Targa) and downshift to brake, but only in normal driving circumstances - never in performance driving and not a corner... the rear engine and RWD will cause a pendulum effect - part of the fun of 911 driving! The 911 also will wind down very quickly under clutch. Other Porsche owners may not downshift for the reason mentioned before - brakes are cheaper than clutches... figure about $100/wheel for porsche brakes, about $1500 - $4000 for a clutch replacement.

2) I downshift my MINI all the time, but that's how I drive (& have driven) all my manual cars.

3) Double-clutching is done when a car has no/worn synchros... clutch in/shift out of gear/clutch out; clutch in/shift into gear/clutch out.

 

Using the brakes will turn on the brake light and warn the car behind you. You might risk someone hitting your rear if you engine brake only.