I suppose "set health 0" and "resurrect" could solve that, but I am searching for a less dialog- and quest-breaking way.

I was JUST about to ask this; namely, the inventory of the creeper.

EDIT// Alternatively, is there a way for me to teleport him to me?

That makes 3 of us. I have multiple weapons/armor combos I sell to one guy, all repaired fully etc, yet they won't stack... namely the weapons.

You can remove the reference of the NPC from your ess file with Enchanted Editor, which will reset him back to default.
This will also reset the NPC's Disposition, but won't break quests or dialog topics.