I've managed to get to level 3 but i really dont understand how holding shift to go through walls works
So pretty much if both characters are next to each other whether that is diagonally, vertically, or horizontally, the one they switch to can go through walls, but they have to be next to each other for it to work for either of them. Also you don't have to hold shift. You press shift and then you switch to the other character. If that character is next to the other character and next to a wall it can go through the wall.
Yes. You press shift, and if the other character is in a 1 tile radius, including diagonals, they can move over walls. When a character moves, they can no longer move over walls. You can walk along walls as much as you like as soon as you get onto one, but if you are on top of a wall, pressing shift does nothing,