RE: Celebi EX and Mew EX
Note the terminology. It says that those Pokemon "can use those attacks"; it does not say those Pokemon "have those attacks". Mew will not be able to copy attacks from a pre-evolution, because they aren't actually attacks on that Pokemon - they are just attacks that are there that may be used.
It's the same reasoning behind why Zoroark Foul Playing a Mew EX (or Mew Prime) can only copy the one attack (Replace or See Off), because Mew only has 1 actual attack.