Some interesting questions. I never read Supreme (art issues) but often fantasize about picking up the Moore issues in cheap trades or quarter bins.
The art is mostly early 90's style and inconsistant but I think is still worth going through for the story. Chris Sprouse does a good job when he was on there, at least.
And I personally prefer the regular issues as opposed to the TPB; the TPB while using glossy paper, looks somewhat grainy.
From what I understand about Moore's run, and based on your characterization, I feel it is safe to say that the entire run, and not just the Amazo homage, is one big unused Superman idea. I wouldn't put too much stock in the differences between Gold/Silver/Bronze-Age continuities --Moore is a shameless borrower from all in the Superman stories he did write, with little regard for then-current continuity (I think that Moore would have disregarded any editorial imperative that pretended the Adult Legion didn't exist, for example).
I doubt the entire run would be a big unused Superman idea from the time period Julian mentions--
The Supremacy, created in SUPREME #41, is supposed to be important to the overall arc (this is including the issues that never got published). Its concept is basically a limbo where all the previous continuities of Supreme go; instead of the Crisis making people non-existant, characters like the Golden Age Supreme, Grim 80's Supreme, Silver Age Supreme, Squeek the Supremouse (a Mighty Mouse homage), etc are just placed there until the next revision.
Theoretically this idea opens up a possibility--if you don't want to use a particular continuity, place it back on the shelf so that others can use it if they want to. I'd think this would be Moore's response to how DC jettisoned a lot of the Pre-Crisis stuff.