The problem with that is that you REALLY don't want to give the advantage away that you are there and armed. Guy could quickly move out of sight and returns guns-a-blazin'.
Carrying two guns is definitely a bad idea. Something you should NEVER, EVER do. Accuracy reduces. Control reduces. Personal safety reduces. Never carry two guns in a ready to fire position with the safety off.
Reactionary decisions are also pretty sketchy to rely on. Again, you lose the little bit of control and advantage you have if a gun is raised and pointed toward you after announcing your intentions. To shoot a gun properly, you'll be exposed in some fashion, so using cover is only a small help. Hearing something and shooting at it is definitely the improper way to act in any scenario anyway, so the college kid coming home argument really shouldn't hold any weight, especially if the kid KNOWS the parent has a weapon the parent will have obviously told the kid he will use it. Kid's got to be some sort of dumb to break into his own house when he knows his parent will shoot someone breaking in.
Another thing you're taught in gun classes is that if you point your weapon at someone, be prepared to use. "Flashing" it is one of the worst things you can do.
Having a gun with rubber bullets seems like a waste of time. Why not just have a baseball bat, instead? Pain is oftentimes not a deterrent to someone hyped up on just adrenaline.
As to the question on what happens if you miss and do unintended harm to someone else, the answer is simply, don't miss. If you have a gun, you're supposed to be trained on how to use it properly, in other words, how to shoot straight. If you miss, you either are negligent with your execution (plain miss), or you were negligent with your decision making (shooting at a side target, moving target, etc.). Now in a struggle or some other circumstance where you're forced to shoot while in distress, than that's a different circumstance, and not really relevant to the scenario being discussed.