I think you're missing my point. I'm not saying "reductionism prevents consciousness from existing". I'm saying that:

If consciousness in the Potterverse is the same sort of thing as consciousness in our universe (i.e. a way of describing electrical signals in the brain, and not a magical ineffable substance like a soul)

and

the differences between humans and animals in the Potterverse are same or similar to those in our universe

then

it is impossible for humans in the Potterverse to have consciousness while animals lack it

and

it is impossible to end death for humans but not animals using consciousness as the criterion for distinguishing between the two.

My original reference to reductionism was just to eliminate the possibility of Potterverse consciousness being a magical ineffable substance (in which case this argument would not apply).