I have been using gibbs for many years. One thing I have never been able to figure out is when cutting a slot with air wall at both ends and using cutter comp is to get the cutter to cut off the far edge of the part. I will cut down one edge move over and cut back on the other edge But the cutter never clears the the part completely on the far side. I either have to cheat and make the slot longer than needed or edit the far move.
On the "open sides" tab, did you increase the overhang?
That should do it, but I usually just extend the geometry like you have been doing.
Apparently I don't know anything, so please verify my suggestions with my wife.