The Omphalous Chamber
There is no indication that Ventari’s Tablet is made from stone that came from space – so I’m not really sure where you got that idea?
As to the second part – it’s entirely possible that the Ventari Tablet in some way allows the dream of dreams to exist, considering that as far as we know it’s possible that the Dream is unique to our tree.
Omphalos is just Greek for navel, or more broadly the center point of anything. The stone at Delphi was called such because the Greeks believed its position the center of the world, not for any quality of the stone itself. We were told since pre-launch that the Pale Tree could be communed with from its heart or center, so I think Omphalos is just a flavorful word to denote that.
R.I.P., Old Man of Auld Red Wharf. Gone but never forgotten.