" So a high resistance short at say 15 amps may not trip the 15 amp breaker, and a 20 amp short on the 20 amp breaker may not trip the 20 amp breaker."
Conversely, you could conceivably get a trip on a breaker when the current draw is below 110V/20A. They're thermal devices. That's why the summer time & hot electrical compartments can be a contributors to popping breakers in RVs.
Fuses can have similar issues.
As you adequately point out, there is nothing that is 100%. Like I said before, I tend to stay conservative and lean on the symposiums of guys with pointy heads that dream this stuff up.
Ultimately, I think the the meat of protection comes down to having adequate ampacity coupled with good workmanship skills.