Silver,
My figure was based on buying the town out entirely for airport / related usage. I'm not sure that you could just 'bribe' people with a payment for noise, although airports do have insulation grant schemes. Either way, that would be a large group of antis to placate, it is always easier to keep noise disturbances in their current place than to move them elsewhere, even if you are proposing a net reduction in noise footprint, the politics doesn't work that way.
And can you please stop referring to these flights as being domestic / Schengen - the UK isn't part of that agreement, and is unlikely to be.
If you are suggesting that a flight transfer between two Schengen countries - e.g ARN-SSI-MAD, then fair enough, but you are still talking about splitting the airport into different zones. The Foster proposal has a lot more in common with HKG - essentially part reclaimed land, part terra firma, and from what I have seen of his diagrams much more of the latter. HKG has two runways, as does KIX, to have a 2 + 2 configuration with central terminal area, enabling sim. approaches should enable the kind of capacity such a project would need (100-150m pax pa), extending to 6 runwas and two terminal areas would nearly double the cost, not to mention the huge extra costs of building in that location.
Sadly, your two other examples are not relevant - Burj Khalifa is a tower, deep piles for a huge superstructure, airports need stable runways over a vast area. The Dutch reclaim land behind dykes, the Thames Barrier 2 would remain open most of the time.