Yea, I thought about changing the attack button from the mouse to 'k' or something like that when as I went to sleep last night. I have to agree, it's a bit of a weird button especially since you can accidentally click off the window.
The different difficulties for pushing the box from different directions is very intentional though. I like that it adds more complexity to the controls (top down vs platformer movement) and makes the player get close walls more frequently so the saw blades can attack and it makes so the player needs to get out of their comfort zone (the floor) from time to time since the ceiling is not only harder to get to but you also can't move on it while for the walls it's easier to get to but you keep sliding down while you are on it. I would actually get distracted while developing this just jumping around inside the box because I liked the double jump so much haha