The substrate issue isn't as bad as you probably think it is. Anything large enough to cause serious damage is usually knocked off during the process of eating, and anything smaller won't really hurt them.

Failed feeding attempts usually result in an "escape attempt" where the snake will look for ways out. It's usually due to the fact that they aren't in the mood to eat and they can smell the mouse and want to get away from the other animal(at least in my experience). Once the smell clears the tank, they usually settle down and go back in their hide.

Regarding striking at the belly, that's actually pretty common, especially while they're eating small mice. If you think about it, they're going for the neck in most cases since they need to hold the mouse still while they constrict. Usually after the strike they move their way to the head. If they're trying to eat it sideways and can't figure out how to eat head first, that's typically a sign that you should look to move up a mouse size.