[Bellerophon]

1. It's difficult to tell if the hop up rubber is worn out or not without seeing it in person, it might be dirty or just incorrectly adjusted. The hop up rubber does have to be replaced every so often, though it can take several thousand shots to wear one out, so unless you've owned that gun for years or are really trigger happy it shouldn't be a problem. Edit: after 5000 shots, the rubber is probably worn out.
2/3. The hop up rubbers and inner barrels or almost all AEGs are basically interchangeable, though the hop up unit (the frame which the rubber is inside of, to which the barrel is mounted) will be different from one gun to another. It shouldn't require special tools to open, and on MP5s it should be pretty easily reached (remove the one pin behind the magwell and slide the upper receiver off, then pull the hop up unit with inner barrel out of the upper receiver).