Andrew,
You have two very good eyepieces there, but they are created differently and have different applications as I mentioned to you on Saturday. The Nikon eyepiece is designed expressly to be use with refractors (convex focal plane, as have Maks and SCTs), so its performance in fracs is outstanding, as you know. But that Nikon is an optical mismatch with Newts (concave focal plane), and the outer edge aberrations you are seeing with your Newt are a symptom of this. Nothing wrong with the scope or eyepiece. Simply an optical mismatch.
That Docter EP is a VERY SPECIAL BEAST. It is rare to find an EP that can work pretty much equally well in a Newt as in a refractor.
As Adrian mentioned, coma in a Newt can be savage, but it is not responsible for the gross aberrations you are seeing with the Nikon EP. A coma corrector won't help with the Nikon EP as it won't correct anything else but coma, and not the field curvature, spherical aberration and astigmatism that the optical mismatch will be generating with the Nikon in your dob.
Also, remember the way we actually use our scopes - we don't do any serious observing at the edge of the field of view. Having a 100° AFOV is great, but when an object is at the edge of the FOV you will move the scope and not examine it closely at all if it is at the edge... This is something to keep in mind if there is a little distortion visible along the edge, and your Nikon will still be a very usable EP with your dob. May well mean that a coma corrector with your f/5 dob is not necessary either!
Alex.
