Yes, digging away that which is not supposed to be diggable is of course a backroute. I could extend the steel area for that part, I suppose. (Or is there anything else that could prevent it? Your description wasn't very clear as to what you do... which edge? etc.)
And yeah, one lemming is supposed to die in level 8, with all tools used. I had a lot of trouble eliminating backroutes on it, so I'm not surprised there are still some.