A couple of quick issues which I noticed using the 2x extender on the 100-400-II-
1) Trying to focus using the LV is going to be PITA. If you do need to use contrast detect AF, then you should start by turning the focus ring manually so that the lens is focused behind the subject and then start the contrast detect AF. This is usually much faster.
2) Unless you have one of the best tripods available, using a tripod with the 2x attached is a no go. Even the slightest of vibration using the tripod is going to muck up the image. I've tried it with my manfrotto (AU tripod) but it is a struggle with the slow shutter speed at f/11. The only way I've got reasonable shots using a tripod with slow shutter speed is with MLU or shooting LV.
3) In my opinion, the IQ using the 2x is better than it was with the 100-400 v1 with the 1.4 extender.
4) My shots with the 2x extender are not exactly razor sharp, but they are not as blurry as yours. I will post some images once I can access my home computer