2) a page "play" for presenting the GC preset where some information is duplicated, and the diagram is often shown with wrong pieces as it uses default pieces, like this one;
I never tried to publish a GC preset, but it seems to me that the diagram that is shown on that page is entirely the responsibility of the user. So if it doesn't show the same pieces as what the actual preset uses, it is because you ordered it to show different pieces.
You could even use an Interactive Diagram on that page, instead of a static one from the Diagram Designer. I recall that I made such a substitution on several GC pages, for variants that only appeared to exist on this site as GC presets, without a real article describing the rules. (Matts Winther's variants are typically presented like this; the GC page links to his own website for the rule descriptions.)
I agree that in most cases this page is entirely redundant, and just an annoyance to the user. There might be a reason for it if the same variant has multiple presets with different piece representation, to give the user a possibility to pick the representation he prefers.
I never tried to publish a GC preset, but it seems to me that the diagram that is shown on that page is entirely the responsibility of the user. So if it doesn't show the same pieces as what the actual preset uses, it is because you ordered it to show different pieces.
You could even use an Interactive Diagram on that page, instead of a static one from the Diagram Designer. I recall that I made such a substitution on several GC pages, for variants that only appeared to exist on this site as GC presets, without a real article describing the rules. (Matts Winther's variants are typically presented like this; the GC page links to his own website for the rule descriptions.)
I agree that in most cases this page is entirely redundant, and just an annoyance to the user. There might be a reason for it if the same variant has multiple presets with different piece representation, to give the user a possibility to pick the representation he prefers.