But saying that shows to Sera that she can get away with this sort of crap, and that doesn't seem healthy.
Sera's not a child who is testing the limits of the Inquisitor's boundaries, like she's some kind of parental figure she has to figure out how much she can manipulate. This is one of her most vulnerable, soft spots, and an Inquisitor who says they don't believe when they do would hopefully be doing it in recognition of that, not just to give in, you know?
Sometimes we compromise ourselves a bit for the people we love. If they're worth it, they don't take advantage of that kind of compassion. I don't think Sera would, tbh, but I can see how a lack of meaningful follow up on that whole kerfuffle might lead someone else to think otherwise.
I understand your frustration with this scene, I do, but I just don't think it's going to be resolved. They didn't put enough into it...not into the scene itself, nor the follow-up, imo, for it to play out in a natural way for the way that I (and it seems like you as well) would play Lavellan.
It's kind of like the scene at the Landsmeet with Alistair in DA:O, except that one makes sense because it happens in a public place, with a ticking clock, and if he leaves, he is g o n e and there is no way to talk to him again.