@samuelchia, thanks for the split files. Here are my thoughts, though this may take a while, new to Gimp...
Some comments. There's the rule in digital sampling that the frequency being sampled must be less than half the sampling frequency. I know the anti-alias filter is there to enforce this, but I can't help think that the "sampling frequency" here is 2x2 pixels and the sky-rock boundary is only a few pixels thick. Therefore there is always going to be noise on the boundary.
Your logic seems to be 1) there are unpleasant artifacts in the final image, 2) you remember there is possibly a weakness in amaze (diagonals), 3) Focus Magic highlights these, therefore 4) it must be amaze causing the problems. I suspect FM is the problem. @cuniek has alluded similarly. The product makes a big thing about deriving a car number id out of a blurred plate, and rescuing terrible exposures, but is it for fine art?
I've put your images in Gimp and compared them, but honestly, I'm not much the wiser (though obviously my untrained eye is not helping...). For others reading this thread, here are some screen grabs of one tiny section from the photo.
1. "Bare" amaze (no chromatic ab. correction, no false colour correction steps). I see nothing wrong with this so far.
2. Add in chrom.ab. and 1 false colour step. Now looking very reasonable.
3. Apply unsharp mask, raduis 1, amount 200, everything sharpened with no halo control.
4. Compare this with your FM result -
(3) does not have the extreme colours and aggressive edge (though it does have an unwanted halo).
The above is why I think the FM sharpening is the problem.
If you want top top quality perhaps a medium format sensor is needed?!
There is a cure for the edge of course. I expect you know this, but for anyone who doesn't, one trick is to take the finished image and put a sharp version layer on top of the unsharpened image. Then with a small soft brush, trace over where sky meets land on the top layer and delete the sharp boundary. This gives a good result since it mimmicks the natural diffusion we see. Here is a larger part of the photo done this way -
Zoom in, I think it stands up quite well.
You will probably say it's not for me to say how you process your prints, and that's fair enough, but we all should be a bit practical and all this Amaze stuff seems a bit over the top when you have a very simple workaround.
@cuniek, I looked at your NEF file, but I honestly can't see anything wrong! Yes there's some purple fringing / aberration, but I can't see these elusive artifacts (except for @heckflosse 's example). What is meant by stairsteps please? Obviously if you make a diagonal out of squares, it's going to look like steps, beyond that I don't get it.
Here is a bare amaze section and then with 1 step, chr.ab and Defringe in the colour tool (as you had).
Again, these sections are miniscule parts of the photo.
Where are the artifacts please?
I know little about demosaicing, I was just interested by this thread. Perhaps Amaze can be improved. But if you did something special for diagaonals, would it cause new problems in some other situation? Where do you stop? - special processing for corners, circles, squares, hair?!