Nice scope!
One thing I can notice from looking a your test shots is what the MPCC mk3 is doing. Some will think its a collimation thing but even with it collimated perfectly you will still find the same thing with the stars. You will notice a bloated look and they are not as small and round as they should be.
With my 10" f4 I struggled for months before finding the problem and it was with the MPCC, I had both the mk2 and mk3 and both had the same problem. When I changed to a Televue Paracorr type 2 everything became perfect and it is way less sensitive to tilt.
Here's an example from my scope between the MPCC and Paracorr. The Paracorr 1 works great too if you could find one used. Even things other than stars look heaps better.
https://www.flickr.com/photos/463028...posted-public/