I don't think it is as much about a magic key as it is about knowing the wards and being home. By all rights, Mouse and Mister count the place as "home," so they don't have to worry about the Threshold (which can be argued is tied to the wards). The wards themselves keep people from opening or assaulting the door (the latter as seen in Dead Beat, the former in The Warrior and referenced in Changes). Bob is "riding shotgun" when it comes to Mister, but Harry's place doesn't have a threshold to worry about anyway.
But the more important thing is this: whenever someone is welcomed in, or walks in, it is clear that Harry takes down the wards. You see him mention it, and he trained Molly to do the same. In Summer Knight, Elaine also takes them down when she comes in.
The Talisman he made for Thomas and Murphy does a similar thing; think of it more like a keycard with a modern security systems if you need to relate it to anything in the first place.
As for the "sphere defense," I think it was one part shock over the power and one part shock over the fact it came from Harry. It seems to be a tough idea that is tinkered with but hadn't had a practical use. I could be wrong, of course, but this does seem to be sound logic to me.
Just thinking aloud, feel free to ignore it if need be.
Two things I thought of when reading through-
-Harry being attacked in White Night is similar to both Viktor Sells and Lydia. Harry attacks Viktor via his scrying shadow by "Sending the magic back up the link" and Lydia allows entrance to Harry's house, bypassing the wards, through her dreaming. So it would seem that Harry's wards protected against physical entrance. Afterall (Changes): His Godmother has been protecting the Nevernever entrance to his house
-Wouldnt the "trap" put on the locker containing the blood samples in Small Favour by Ms. Gard count as a ward with a landmine effect?