Hi, yes, these rules have been used with my group - so I guess they are playtested. I agree that the killing blow for XP is arbitrary, but that was by design to add additional tension. You can easily not use that rule and divide up XP otherwise - remember that they don't get XP for weaker monsters, so even of they kill a horde of 0 level Kobolds it doesn't count. I'm also not worried about power creep like in 5e since Shadowdark has a different level progression and HP system -- basically its fatal enough combat that I don't worry about power creep as a GM.