1, they don't come for you, even from the MP40 window.
2, it does make you invincible basically, as you can leave the room and come back 30min later and you will be rounds farther. I only do this on solo for youtube vids. So I can show my friends at school that i make it to round 50+.
3, dogs are affected by this glitch, if you do it right.
4, zombies only come up the stairs if you're doing it wrong.
5, i have done this glitch a few times, only on split screen or solo, so no leaderboard glitching.
and 6, i encourage glitching if you're doing it to show off to your friends, just please don't do it online, for it will make you get farther by cheating, which isn't fair.