In Plagued Mine you need to hit a switch to get the gray key, and the gray key is needed to open the gate for the red key. However you can get it early and let me explain how. Collect 70 coins on the way to the store and buy the tongue extension. When you arrive at Plagued Mine you go directly to the switch, jump off the wall and bounce on the bat that drops bombs. do 3 bounces and stick out your long tongue to grab the key. Then just head to the red gate. This saves 20 seconds and makes 18:xx possible.