Are there any other things that could be a problem beside hiring or buying a plane for the service?
Generally speaking charterers of a long-term contract prefer not to go for 'one hit wonders'.
From what I've read on here Atlantic Star don't actually even have an aircraft, never mind a fleet of aircraft, of their own, that they coerced a somewhat geriatric BAe146 jet in to St Helena that would need adapting, modifications, to operate any SH route and even then with reduced payload.
Were I SHG I would be questioning:
1. What is your back up plan in event of any technical problem and in particular regarding heavy maintenance checks, and:
2. What is your plan for the future once BAe146's are retired to their graveyards?
On the other hand Embraer went in to SH with a shiny new jet and, again from what I've read, there is an operator to acquire a few of these shiny new jets.
If there were only one bidder of a single geriatric jet then desperation calls for desperate measures, but should there be two or more bidders then .....