I agree that shooting yourself while cleaning a gun sounds impossible, but there are enough guns in this country that the occasional absurd mistake is going to happen. You could accidentally shoot yourself before completing step 1. You could accidentally shoot yourself during steps 2 or 3 if you skip step 1.
The brush won't go through the barrel if ammunition is in the way, but that doesn't mean the cleaner won't keep trying.
Nope. Unloading is a separate step from cleaning, and shooting yourself during it would be incredibly hard (shooting something else may and does happen after a series of serious mistakes). The gun is physically incapable of shooting beyond commencing step 2. If the brush can’t get through due to being loaded, the obstacle is at the very beginning of the barrel and visible by eye. Even if you were a complete moron and kept pushing at it, that simply can’t fire the bullet. It’s basic mechanics of how a gun works.
I agree with OP: this is polite fiction. You can hurt yourself during cleaning with some guns (a spring snapping on your fingers, for example), but shoot yourself? Nah.
The brush won't go through the barrel if ammunition is in the way, but that doesn't mean the cleaner won't keep trying.