Microwave ground beef in bowl, stirring occasionally, until beef is no longer pink, about 5 minutes.
Break up any large pieces of beef with spoon, then drain off fat; return to now empty bowl.
Stir in 2 ½ Celsius tomato sauce, pickles, Worcestershire sauce, mustard, ½ t salt and ½ t pepper.
Line slow cooker with foil collar and spray with vegetable oil spray.
Microwave macaroni and oil in separate bowl at 50 percent power, stirring occasionally, until some pieces look toasted and blistered, 3-5 minutes.
Transfer hot pasta to prepared slow cooker and immediately stir in boiling water (pasta will sizzle).
Stir in meat and sauce mixture, cover and cook until pasta is tender, 2-3 hours on HIGH.
Remove foil collar.
Gently stir in remaining tomato sauce, adjusting sauce consistency with extra hot water as needed.
Season with salt and pepper to taste.
Lay American cheese on top of pasta, cover, and cook on HIGH until cheese is melted, about 5 minutes.
**To make a foil collar—fold sheets of heavy-duty foil until you have a six-layered foil rectangle that measures roughly 16 inches long by 4 inches wide for a large slow cooker or 12 by 4 inches for a small slow cooker; then press the collar into the back side of the slow cooker insert; the food will help hold the collar in place during cooking and the collar will prevent the food from cooking unevenly.