Steve,
At the end, you may find your lens is not good (enough).. but I would definitely try to see if mirror is pinched.
I have similar (not so good) experience with
this one (Samyang, Opteka, Whatever).. It has coma in the corners. I know there are good specimens out there (from searching Astrobin), but mine is like that and I will be using it for guiding..
Also I intend to use it for Sun eclipse in 2028 (provided I'll still be alive and kicking).
EDIT:
One more thing - I am using Canon cameras with APS sensor... so if your Nikon is full frame.. it will have worse results in corners.