1.They didn't try on an immune person, so not a real point. Not sure how 100% success is any more said in 2 than 1, maybe show me the contrasting evidence? The dialogue in 1 supports the idea they would be successful, same in 2, neither one contradicts the other, please bring specific examples to prove otherwise.
2. In order to be wiped out there'd have to be no one left to rebuild, in the game's narrative they don't get "wiped out" it's just without leadership a lot of people LEAVE and the power structure fractures, some of them make up the WLF, for instance, but some decide to keep the Fireflies alive, that's not wiped out.
3. "straw grasping" or grasping at straws is what I call someone claiming "they could grab sharp objects" by that logic you should be forced to kill the other two nurses as well, right? So, Joel killed the one to terrify the others into submission? Sounds like a swell guy and not a monster. The point of picking up the scalpel is to show Joel how much it meant to him, saving the human race. Joel no more needed to kill him than Superman needs to kill a thug with a gun. No redeemable qualities? It's absolutely morally bad to sacrifice one to save potentially millions? This would require an extensive philosophical debate but rest assured you are not the arbiter of what does or does not make someone a bad person.
4. Stop using this tyranny of the author bs, it's not a real argument and doesn't mean anything. It's not author tyranny that Joel acts in character. It is IN CHARACTER for Joel to murder those people in cold blood for Ellie, it is IN CHARACTER for Joel to do it regardless of whether or not they knew for a 100% certainty she could cure everyone. The excuses you make for Joel are not excuses he'd make for himself, he knows what he did and it haunts him even if he'd never change what he did. You're own inability to cope with Joel's decisions makes you look more emotionally immature than a video game character who doomed the human race.