That seems to be a really weak jump of logic.
A Path she doesn't know exists, that comes with its own strings attached is her preference despite her not voicing anything to that effect before hand?
Tranquility seems to be the preferred method considering she actually endorses it, she doesn't even mention possibly saving the half blood as a priority, only that he not be possessed.
Well based on the logic that we don't know about future I listened to advice of Anders and a fade entity who know about this matter more than anyone. It seemed logical at the time. And we are talking about the person that will turn on you to save feynriel and any romance will end regardless of anything. With this determination I believed Feynriel could be saved without knowing we could actually save him. (I thought we have to tranquil him on my first playthrough)
Mmm, Now that's a funny word it isn't it?
Conscience, Soul, How much are those worth?
When you stand behind people and the abyss, between order and chaos, what is personal morality worth?
How many innocents is a mage life worth do you think?
I attended a class some time ago. Its name was "gray morality" The professor who was also a Judge showed us a real evidence. there was a car accident and the bus driver tried to save her wife but in doing so he caused a hundred people to die. Now if you are in a situation to save one person or a hundred person, which one you would choose (forget about personal feelings for now, only the numbers matter). Several years later the wife who was a doctor invented a revolutionary drug for curing cancer at early stages and ended up saving thousands if not millions.
Now what are the chances of such person appearing from those hundred people who died? I'm not implying anything, all I'm saying is its not as simple as minority vs majority. We should try to find a better way and your ideas doesn't seem like trying. You know the mage that helped grey wardens become what they are might have died if someone didn't save him and without him we wouldn't have grey wardens and world might have ended or many more might have died. Every life deserves a chance, if it fails that chance that is another matter. All life is to be treasured but to protect life, there are times when we must destroy that which seeks to end it. Destroying a mage for something he might do is not such a time, its simply paranoia.