Why is the Backup Battery switch not connected to the battery?

"A failed master switch or turning off the master switch will not stop the engine"

Ok, this changes everything. I genuinely did not know that. I guess, nevermind then. :)

It is folly to initiate work-around solutions to perceived scenarios.  Anything you can think of happening (and trying to fix or prevent by altering a proven and reliable system) is equally as possible as anything you don't.

If you follow the wiring diagram, your engine will start.  Once it starts, it will run independent of the battery.  Go fly.  If it fails in fight, and won't restart for whatever reason, prior preparation, training, and proficiency in emergency landing procedures is a much more productive use of your time than attempting to design a system which overrides known safety features, and may induce unintended consequences which increase your risk rather than limit it

No matter how hard you try, it is impossible to eliminate all risk.  Any deviation from the tried and true in an attempt to do so is conducted at your peril, not the opposite.

Things are the way they are in aviation because the manuals are written in blood.

"attempting to design a system which overrides known safety features"

Wasn't trying to, was just trying to understand why the system seemed less safe than it could be. It appeared at first that a failure of one switch would stop the engine and prevent a restart. I was a bit ignorant in that area, I asked smart people about it, I was informed and corrected, I learned and reversed course. ?

My system is now wired per spec, and I'm prepping for first engine start this week.