It's funny how you justify Messi's inclusion by bringing up an injury time goal in a league game early season....But that still doesn't comply to your selection criteria.
So Messi is an exception to the above?
Murphy has taken them under similar pressure for his team. Elano is another one who did it while he was at city and i don't think he's ever missed one yet.
As for Messi, he used to alt them with Eto'o before and now does it with Zlatan. He has also missed a few. Neither has he taken them "on the biggest possible stage under lots of pressure"
Messi is one of the best players in his position but he certainly isn't one of the best at PKs yet. Only the very biased Cules 'll think otherwise.