Thanks for the crops, I agree traditional star trails. And with that much trailing, the milky way should be slightly blurred when you view it at 100% on a single exposure, since the trails will be in tack focus and the movement of the nebula makes it hard to view, as your eyes will focus on the sharp trails.
And sure you can use the 20 1.4 wide open for traditional star trail photography and the coma will not show, but since I tend to work with partial moon for illumination, wide open is too much.
The Milky Way, is another subject totally, and much hard to work with for sure.
The method that Adam Woodworth uses, combines higher ISO work, with F 2.8 glass, or higher and he is using even ISO 12800 on the D810A. The best Nikon for the Milky way for me is the D750 as it's higher ISO range is 3x cleaner than the D810, per Adam's research the D810A does much better but it's just to expensive for me to justify and the camera won't work well in normal lighting.
http://www.adamwoodworth.com/ I would love to travel to some of his spots.
I agree with all your points on faster glass over tracking hardware, even though as shown by Graham, the tracking hardware can produce wonderful results. And the difference between what you can capture at F 1.4 and F 2.8 is also impressive. I have just resorted to cropping out the edges with shots from the 20mm 1.4 and using the center, also tend to use the 4:3 Nikon D810 crop when I have that camera on, or on the D750 as it takes out the vast majority of the areas with coma. There is also a coma brush you can easily make in Photoshop, that will do a wonderful job of removing the wings, just takes a ton of time, but works good on the brighter stars which display the greater amount of coma.
Sad to see the Sigma off center, but that is common on their lenses, as I had to purchase 3 24mm 1.4 lenses before I found one not off center on one side or the other and the lens I ended up with is still not quite perfect, but gets the job done.
The other issue of course with the F 1.4 is the extremely shallow DOF exhibited, so working in a combination shoot, attempting to pull in parts of the surrounding landscape can be difficult with out a focus stack, but it can be done.
All this just points out how much work is needed to be done to really capture such a shot, and sadly the vast majority of viewers tend to take it for granted and or feel it's faked in some way (general perception of most digital photography now).
Paul C