Something to remember about hardening and softening Leliana: both times we see it happen, she is in a vulnerable emotional state, thus easier to influence. With the Warden, it happens after the meeting with Marjolaine, which troubled her deeply and with the Inquisitor it's after her mentor/friend/surrogate mother's death. Of course her personality seems to flip on a whim. She feels utterly lost in the world, abandoned by the Maker and wondering what to do. Let's not forget that people were calling the DAI protagonist the 'Herald of Andraste' at that point. Who's to say Leliana didn't believe it too? It would explain why the Inquisitor-to-be had so much sway over her so early.
Remember who is the first to join in 'The Dawn Will Come' with Mother Giselle: Leliana. It's not the first time she's received a sign from the Maker. (the rose in DAO)
On a side note: let's assume the Maker is real for a moment and He does have a grand plan. That would mean that wholesale slaughter at the conclave was necessary for things to happen as the Maker wanted. Is the Maker a blood mage then? Discuss. *runs away*