I dont remember the priestess having elf ears and cat like pupils though.
You are correct, she did not.
But, what if after he was beheaded in her world she went insane and gained a hell of a lot of power as result and transformed? Or she dabbled in dark arts and the consequence for that was a change in appearance.
We don't really know if that is her, but it was definitely hinted at that she and this new female demon lord might be similar in some way. I have been thinking that the person in the armor is his body reanimated as well.
To the other person's point, though, I don't know why they would bother putting him back in his old body when he still has all of his powers from when he spent time there AND the demon lord has the ability to transfer him to their world. The demon lord was probably just waiting for a bit and now that he was there decided now was the proper time.
Or it could be that all of this is just bait for us to think that.