OK, so for the map, I can try to put something together. Given that the hallway is only 5' wide, I don't think more than one PC can be in melee with the target at a time. I would imagine that would be Colden since he just got bit. If that is incorrect, let me know.
In that PM I sent, it basically contained the only references I could find to passive perception in the Basic Rules (p. 60, I think?). As for how I'd play it, if the PCs don't tell me specifically that they are trying to notice something, I would just use their passive check versus whatever the opposing DC or enemy stealth check is, but that would work both ways with PCs and NPCs. I think you just have to watch out with allowing Taking 10, otherwise the party rogue is almost always going to beat a guard's passive perception score with their likely Expertise enhanced stealth check.