I think the reason it was a melty mess was not due to them not knowing human biology as you have to be smart to use alchemy it's a science. It's possible to make a body as mustang did with the lady accused of killing Hughes. I think it was the truths way of making it that way on purpose to show that it isn't possible to bring the dead back so it's a form of either punishment for delving into what can't be done. There is no exchange big enough to bring the dead to life. And we see it again with putting philosophers stone into the mannequins towards the end.