About the key: the locksmith would go to the office and either take the lock apart and work out how to grind a new key, or put in a blank key and put turning pressure on and the marks on the blank would show what needs to be ground away to make a new key.
The mystery to me is why Greenleaf would make it look like he was being framed and put himself into the situation, when all he had to do was make the false alibi, then nothing.
I think Greenleaf wanted to frame Eddie Kane. Columbo said to Riley: "somebody has a copy of that new key, and when I find that key, I have my murderer". So Greenleaf had a new key made and planted on Eddie Kane. The problem was, Columbo was setting him up from the beginning.