Well this would make more sense in every way. Yes there was duct tape on the skull, but that doesn't have to mean it was on the mouth. Since duct tape keeps it's stickiness, I'm sure it's very possible the duct tape was somewhere in that bag (on hands, mouth, etc), and then when the body decompsed and the bag was being all jumbled around in the water, the duct tape could have been all over any bone...
I'm also thinking the duct tape could have been used....ugh this is hard to type...used to keep the maggots from spreading. If they were coming from her nose or mouth (or anywhere), casey could have put duct tape there to keep them from continuing to spread.
Speaking of maggots though...Let's think about a bunch of things here. There were cadaver hits in the backyard. So do we now believe she did try to get rid of caylee back there and then moved on to just dumping in the woods instead? What in the world could the shovel have been for? Maybe the trashbag wasn't just thrown out there. Maybe casey did bury it a bit but the water washed it up. I don't see her working too too hard to bury it 6 feet under, more like a little ditch and then go about her way. This would explain how she had the shovel back in such a "timely" manner, she was close to home. Would this still make it long enough that caylee sat in there before she was disposed of? Does that timeline work?
Just thinking out loud...