sunday8pm
I don't know the answer to your question about Luanda.
Another problem with your suggestion is the issue of duty hours. Would the same crew be able to fly out and back? If not, wouldn't they be faced with carrying a spare crew (not sure of the regs on that) or alternatively slipping crews in St Helena who would then have to wait for the next flight in (say) a week's time? That would be expensive, and there isn't much hotel accommodation on the island anyway...
I'm not trying to be negative, but the peculiarities of St Helena's location mean that a lot of the otherwise sensible suggestions made on this thread aren't really practicable.