I'm having a similar problem, I can't seem to get the delete option to work. Every time I use it the object can no longer be rendered in anyway, I get an "instance does not exist" error and the object remains one solid piece but the whole piece can be moved from front to back. I'm editing shatterhand, on 2 stages there are things in the background that won't separate and others where I'll edit a piece in the background but if a similar part that looks identical to it in the foreground, even if it's a different color it pop out into the foreground. On the under water portion there are sections where 2 different pipes intersect, 1 in front the other is part of the background but it seen as 1 piece and delete does not work o it