I thought it was just me, but yes the key is a bit out of reach strapped in. Instead of a key extension maybe a kill switch mounted on the steering wheel or trans tunnel would be better.Sorry to hear about the incident. AutoX has always been very safe. Accidents there are 1 in a million. My thoughts are with the injured.
When I road raced, I always took time every year to make the car safer. Racers being competitive tend to ignore safety. It's good that you're looking to improve there.
In my opinion, a stuck throttle is the big concern in auto-x. Practice what you would do. Clutch in (eff the motor), switch off. It's hard to reach the ignition in a Mustang when you're strapped in. Maybe make a key extension. I'd bet that you've seen them before. But be careful not to turn it into a spear. Return springs everywhere. Carpets can snag things.
Brakes are all about maintenance. Replace everything every 5-10 years. Hoses, lines, cylinders, etc. It's easy to let these things go long in a dedicated auto-x car. Chose good quality parts from known suppliers. You know this. Test after your changes.
A hoop with a harness bar works great. But remember, this keeps you further from the key. The additional stability will help your times.