A strange proposal. The Falcon has the range to fly from/to Windhoek without a 'political' stop in Walvis Bay. But Windhoek is not really a hub. Air Namibia only to Franfurt and Condor is only a seasonal 'leisure airline' flight from Germany. For a flight to the UK three airlines would be involved, makes flights expensive.
Taxes and surcharges also have to be added.
The Falcon will be cramped with 19 seats - nothing like the photo in the St. Helena Independent. Limited baggage / cargo only.
The operators have no reputation, an artificial consortium. No background to support tourism development on St. Helena.
Using runway 02 question remains, up to what level of tailwind doable ?
Using runway 20 how wide and high is the 'windshear area' ? Can that turbulence really be avoided by touching down 300 metres further south ?
Theoretical assumptions, I believe they are shy to extensively test that as deemed too expensive.
I would be very surprised if DfID and/or SHG would allocate a 1.9m GBP subsidy to such a project.