I've found a way that i can reliably get from layer 1 back to the bottom initial key chamber opening layer, using the groove/slot in the top layer, but as yet i've not managed to make it down the layer 0.
I'm convinced that I need to use the slot, as there doesn't appear to be anything else in the layer to use (unless someone can tell me different?). I have wondered if the slot is actiually a route to a secret layer above layer which you traverse through to get to layer 0 - If it is i've not found a way to get there yet.
I came so close to cutting mine open last night - another forum member persuaded me to just put it to one side for a while. Trouble is i know once i put it down i'll never pick it up again!