"Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends."
"I wish it need not have happened in my time," said Frodo. "So do I," said Gandalf, "and so do all who live to see such times. But that is not for them to decide. All we have to decide is what to do with the time that is given us."
...if another player previously used a merlin card to remove a standard black card from, say the holy grail, can I play another merlin to remove the second last played (which would now technically be the last played)?
Yes, but only during the Heroic Action phase on your respective turns.
BTW- this was also answered previously.