At this point, I cannot tell if everything you've stated in this thread is based on experience, or if it's all based on video games and things you read online. I grew up hunting in NY. Until a few years ago, the ONLY gun you could use for deer was a shotgun with a slug. The vast majority of hunters, at least in my experience, used the regular bead sight. (It was easy to tell who got the new scope for their 12 gauge - they had a black eye.) I'm not sure how long your hallways are, but if you can't regularly put the bead on a target at 20 yards - while experiencing the same adrenalin rush (big buck vs. home invader) - you shouldn't be in the woods, and if it weren't for grocery stores, Darwin and evolution would kick in and let you starve to death.
At in-home distances, even with a long hallway, shot spread isn't significant enough to make a difference. If you're not on center of mass, I doubt a few pellets in an arm is going to stop a determined intruder any more than the racking of the pump. Shot that will deeply penetrate, such as #00 buckshot from a 3" shell is going to sail right through walls, capable of severely injuring someone in the next room. Or to put it another way, shot and slugs designed to kill a deer, and by extension, probably appropriate for killing an intruder, is capable of going through walls quite well. Shot designed for shooting targets, killing squirrels and bunny rabbits - much less likely to be lethal; though, I suspect but am not certain it would also get the job done of stopping an intruder.
And, lastly, when it comes to aim - <speculation at this point> I have minimal experience with handguns, but I would think that shotguns are a heck of a lot easier to aim than a handgun, though slower; the aim of a handgun being something that could only be remedied by repeated practice at the gun range. (Or, for people like me, practice in the back yard.)
Oh, and lastly, slugs - they go in one side of a deer and out the other side. Where they go from there, I don't know. (Rule of gun safety: always know your target and beyond.) If penetrating walls of your house, and then the neighbors house is an issue, slugs aren't the way to go. My personal best guess is that steel shot would be the safest for protection of next room/outside of house.
And, my recommendation to the OP: if you're not going to practice at the range, then shotgun for the reason of aiming. It's a bit slower to aim due to its inertia, but it also ought to negate the need for 9 shots.