@Longshot: I'm hardly one to criticise anybody for essay writing on forums, or for taking essentially meaningless discussions in deadly earnest, because i do both and have very good reasons for why... however, in this thread i'm convinced you are being intentionally obtuse for the sake of 'winning' an argument, which is fine on a forum dedicated to, or with a predilection for, pure rhetoric, but feels unhelpful here, even in a daft thread...
I think it's fairly obvious that Ed intended to use his katana/doorway combination in the only sensible way - while standing just inside the doorway, with enough room inside the room/hallway/etc to swing and/or maneuver a standard length 30" katana, he would use its length to either stab the only zombie that could get near him, through the doorway, through the brain, or slash if the stabbing proved impossible. Any defeated zombie would fall where it stood, right in front of and in the doorway, creating a trip hazard for those coming behind.
I have reservations about an untrained person's ability to successfully stab a moving head sized target and recover a 30" flat sided blade, however taking the zombies' complete lack of self defence into consideration, I think that on balance that 1st zombie would not get through the door. Thereafter, due to the trip hazard, Ed would have sufficient time to use a foot to brace the body while he recovered the blade, but he would require good luck to bring it to bear on the 2nd zombie while still close enough to the doorway to pile the 2nd dead on top of the 1st...
I think that stabbing through the mouth would be effective. It would allow the severance of the brainstem, either in the 1st thrust or, partially acting against the mandible as a pivot, during recovery.
I have been attacked with a baseball bat twice in my life. Neither occasion went that well for me, but obviously I did not die, and, you'll have to take my word for it that, the only reasons I came out of either encounter without sufficiently grievous bodily harm to prevent me from taking up airsoft years later was because, despite the injuries I did receive, I remained enough of a threat to my attackers with my bare hands to decide that they had made their point...
I don't feel that I could have done anywhere near so well against a sword.