to what extent do you believe estella when she says no heart, no softness, and no sentiment?

Estella has been raised to feel nothing, and you need to understand that in being raised by Miss. Havisham she has never been treated with any kind of emotion. I think that because of the way she is raised, it's not so much that she has "nothing," but that she doesn't even know what these things are. You often cannot understand things that you haven't experienced.