Wait, wait, wait, I'm not knocking house rules - what I'm recommending is to use very, very simple house rules, that way they are more likely to do what you want them to do. You seem to be having complications right now and my hypothesis is that it's related to their complexity.
This is where I feel you are wrong. I CAN just say that the characters don't die, I can roll dice behind a shield and get a Nat 20 and go..."Um, wow!!! He just misses you!" That's not my style. My rules might be a toned down version to the original rules but, I can accept the results easier. I don't have to fudge for characters or NPCS, I can use the rolls from my Optional Rules without wincing.
Wait, wait again! When I said "Can't you just say characters can't die before X amount of time?" I mean say it
outloud, to all the players - thus it becomes a rule (house rule).
If that doesn't seem realistic, recheck the note I gave previously about that.
As for the extra stuff at the end of your post, that seems to be trying to cover some other stuff other than 'keep PC's alive until X time'. To many bases to cover at once, so I'll focus on the PC's thing for now.