See, and that's what I've been trying to look for is that distinction. I couldn't really find anything specific on a line of difference from them. Which brought about this questions. Also from past research in other topics, what I have noticed is that this game is very bold on what is not allowed. So I've come across the assumption is that if the game doesn't state that you can't do it, then that brings opportunity for creativity and ingenuity.
Now I get what you are coming from with the other two spells but from reading the spells the only comparison you have is the Target requirements. Which is "one dead creature." Also from reading the spell Speak with Dead it specifically states it can't be used on a corpse that has been turned into an undead creature. Same with Raise Undead, it also states that it can't be used on a creature that has been turned into an undead creature. Sculpt corpse only states that spells or effects that target the corpse under the influence of this spell treats it as if it still had its original appearance. So it never really says it can't be used on an undead creature like the other two spells it gives as example to it's statement.