Hi Nick - add my thanks to you for this link and your analysis... makes one think! Whatever the answers trun out to be (and I suspect Mars has some surprises in store for everyone), opposing viewpoints are always energizing and interesting.

Might the 500 m of ice represent most of the ice/water that was ever on Mars, with the Deuterium enrichment only happening with the water right on the top of the ice layer? Since most of the rest of the water (however much it might be) is covered, might it not have "normal" (Earth-like) H/D ratios, with only the surface/near surface parts being enriched?

Are there any mechanisms whereby relative surface elevations could have changed enough that the ancient, cratered areas you described were once high enough so as to be above a past ocean? I know that a thick overlay of ice on Earth can cause the underlaying crust to become depressed, but of course Mars' internal structure is much different from that of the Earth.