Co-op is already included in the demo! The player 2 controls are shown in the game's boot up screen, and by default the 2nd player can insert credits with 'N' and enter the game with '2'
About the amount of stages, the goal is to make 6 of them, but it's possible that it might change in the future
Yeah, I seen this video and its a nice way to show what theco op playthrough is like though I wish many poeple will post co-op videos of Aqua Ippan without death and no miss, now there are msotly single player mode videos , if the game reaches to its final version, we hope we can see a perfect co-op game play in the future
by the way if you find a bilibili of a co-op video show us the link.