Michael, the invert tip is a good one. I tried to figure out how to do that earlier.

I am getting a strange extrude when I group the curves + extrude; edges are missing in a couple of places. See the pic. You should be able to use the curves enclosed earlier to try duplicating this condition. Orient the axis as indicated in the LL corner. I rebooted Moi and Windows and then recreated the extrude and the results were the same.

L.