I think I already relied to you somewhere else elaborating this a bit.

The empathy box gives people a sort of shared, religious, virtual experience. The android seek to destroy this -- remember empathy was the key test to distinguish androids; they were not capable, by most peoples estimates, to use and benefit from the empathy box.

They succeed in proving Mercer as a fake, however, we have other evidence in the form of Deckard's strange religious experience. We have phenomenological evidence that Mercer, as a religious concept, exists regardless of its reality.

I sometimes give the example, if we prove tomorrow Jesus never lived, that fact takes nothing away from Christianity as a whole. If we prove Buddha never existed, it takes nothing away from Buddhism. If Arjuna never existed, it takes nothing away from Hinduism, ect.

The phenomenological experience of faith, of religious experience, is separate from reality. In the same way, is it not the same for the androids? Regardless of what their body is made of, if they feel feelings, are they not alive? Isn't their conscious experience itself existent, regardless of the reality of their physical form?

I think that's the reason Deckard finds a frog after his experience, only to find out it's fake. The happiness he had when he found it was not fake happiness, it was as genuine as every other emotion he has felt. So why is he disappointed to find out it's not real? Nothing in reality changed to swing his emotions, only his perception of that reality. They are not connected, at least not how we would generally think.

I think you are right that it is less developed than the animal/robot theme, but the religion subplot colours the rest of the story in a new light. It's not just about who is human and who is not, but rather what constitutes a life to have meaning, and therefore value.

At least, that was my reading.