The KO count actually maxes out at 9,999 kills. I managed to get it up there at the Battle of Chibi...my game glitched, and massive waves of enemies continously spawned in this tiny little area.
As for the co-op, I was slightly disappointed with the limitations. When it comes to local play, I think I had more fun skirmishing in DW6. However, playing online with friends in other countries was a lot of fun.