Pfft, you don't need American cheese to get a proper melt, you just need a bit of patience and timing. If you really want an easy melt, throw on a jack cheese, like Monterey Jack or Pepper Jack. But if you cover your grilled cheese or burger, even a harder cheddar will melt well, you just need to eat it while it's hot.

For mac and cheese, you just need to thin out the cheese with some half and half (or milk if you're hard up). Make a roux, then add your milk and cheese, then mix in your noodles. The consistency doesn't come from the meltiness of the cheese, but the milk/cream you add to the roux.