Well, I solved finally the problem... the hard/tricky way.
I found this thread
on the Spellforce Official forums on how to hack items into the inventory, so I just messed around a bit and got the missing head.
Given I killed the guy in a legal way and didn't get the reward (the head xD), I think it's all fine now between me and the game :V
Thanks anyway. Hope someone would find this solution useful ^^
EDIT: Can't I mark my own answer as a solution? =/
EDIT2: Oh, I can't belive this. after all the fuss with the editor, while looking for that Einar guy to give him the heads, I found the real head behind a fence, with a dead goblin next to the corpse. Oh, lol...
Now I only have to find the Sergeant Einar...