It isn't possible once again to talk her down under certain circumstances, and said circumstances also involve Leliana not being hardened what is factor that is made before not after she decides to kill you if you defile ashes, thus killing leliana is necessity as you have no other option under given circumstances.
I have never said that in first place that you couldn't kill her (at least not in this thread), well despite if we are being specific you can't because you can't kill her only injure her in WH.Also hardly, it was rather obvious for players she is alive (unlike Leliana) considering we never saw her death only inflicting not very lethal injury that you could recover from and said person was mage.
Well in regard of Leliana i personally always found the ashes as to be more useful than the blood offered by someone that i couldn't even trust(from a more realistic point of view you know that Kolgrim has reason to want revenge against you, so he could have just prepared a trap and poisoned the blood to kill the Warden) plus the spec is available without
kolgrim as well and can be obtained later in DAA or simply by reload a save.
So there is no reason gameplay wise to want to kill Leliana.
Morrigan cannot be attacked until Wh and there the attack is very much lethal and is similar to the attack that Duncan did to Jory, but unlike Morrigan he was with an armor but yet he died.
Morrigan there was granted of plot armor since as others already said is the Warden who push her into the mirror and later simply decide to not cross the mirror (which was intact) to see where she was.
In DAI also i still did not have understood as for why the dragon of Mythal is killed by the red lyrium dragon( after the terrible fall from the sky) but Morrigan was fine (if she was the one defeated by the red lyrium dragon)