Maybe if the players who attacked first took a screenshot to show the progress until the "champ"/another random player goes for the last kill, it would make things easier for the staff. HOWEVER, it seems impractical to screenshot every MvP you kill.
It's not like I support the idea that the last killer deserves the prize, but there are just too many complications to decide who deserves what in the end (can't really divide the prize (probably the card) equally, etc). Sadly, nobody kills for the thrill anymore D:. There may be too many problems for the staff, so I still guess the rule shouldn't be included. It's just a matter of sportsmanship and player integrity.
But yes, the rule mentioned in the first post should be re-worded if the staff really wants to have that concept in.