For cars with a push button parking brake, is there a universal method to unlock the parking brake, eg when a crucial circuit board malfunctions?
No - there's often a way to release it, but it will depend on the manufacturer's particular implementation.
For cars with a push button engine start and steering wheel unlock, is there a universal method to unlock the steering wheel?
No - since the steering wheel lock is at least partly there for anti-theft purposes such a method would defeat the purpose.
Whenever I park front in into a parking spot in a parking garage, I have a dreadful feeling about what would happen if I am unable to start the car. How am I going to get the car out? Is this a real concern?
Yes - but not a particularly big one, since the most common reason you wouldn't be able to start, wouldn't be able to release the parking brake, and wouldn't be able to release the steering lock is going to be a flat battery then a battery jump-pack or a nice long set of jump leads would seem to be the obvious solutions to that concern.
I've been driving for over two decades - and owned cars with technology levels ranging from early nineties basics where the most complicated bit of electronics in the whole car was the AM radio to a 2011 car that has an electronic parking brake and more electronic control modules than I count. I've never once found myself in a situation where having a manual handbrake or steering lock has "saved" me.