I have successfully stitched your images with both PW7 and PW8 with no signs of a zenith star or pinching effect.
This was after correcting the Zenith shot (7.jpg) to match the orientation of all the other shots.
You are doing some weird things to your images and this is not helping.
- Turn off auto image rotate in your camera.
- Stop whatever HDR thing you are attempting it just isn't a good idea.
- Maintain the "exact" image size and orientation with all images. (Currently they are all over the place in size!).
I think if you resolve the orientation with the Zenith shot you will likely overcome your main concern.