Most mice typically tend to land on the small platform between the two pits and wait for the Shaman to build a bridge. Other mice wait for the Shaman to build the bridge and then jump down after. It's also possible to jump to platform with cheese, but it is not easy. To do this, you must rely on the fact that even light wood has small restitution (around .03). You have to run off the high platform and bounce on the right corner of the small platform and air jump to the cheese platform.
Once the Cheese is retrieved, mice can either wait for the Shaman to build a way up or, if they're experienced, they can Wall Jump their way back up themselves.
Note: The wall is reachable even with cheese from the miniature platform in the middle. Note: Something most players don't even know is that you don't have to corner jump to get to the cheese. This won't work if you are lagging, however. Just run across, and don't even stop or jump, and if lag doesn't hit you while running, you should be able to get the cheese easily. Something not even pros have realized.
Most shamans just build a simple plank between the pillar in middle and cheese to let mice across and then make a bridge up to the hole or across the gap to allow wall jumpers in.
Expert shamans also like making a bridge to close the gaps and then make an scissor elevator back to the hole, although it wont go all the way, some Mice use it as an advantage to get firsts, Sometimes it isn't needed because as the gap closes, most mice will start walljumping to hole.