Wait, if karthus has comet and he ults...

No. It goes to the target closest to you. You can test it in a target game with dummies and put them around the map. The comet will always go to the closets one.

Try it in the practice tool and place 5 dummies. It would be neat if it fired comets at all 5

IIRC person at top of scoreboard, same as greenfathers used to.

People always say it is the top of the scoreboard, but I recall a rioter saying it is based on who connected to the game first for the enemy team.