I loved this game! Escpecially since I'm a picross fan. Picross galore! The multiple solutions didn't really affect me though. And for the hotkey for a whole line to be blue/black, you just hold down the mouse and drag over the line. 5/5 and favourite.
the hard badge just says to get 1000 feet in challenge mode, not neccesarily in 3 days, so after ur done, you can load game on hatchback challenge on day 4. Not sure if it works though, cuz i already got the badge, but worth a try.