It's over to Easyjet now and they market the new services. Easyjet certainly is the right airline. Though I must say am surprised that Easyjet have set up a new base at an airport that is how can I say 'not tried and tested', though I think that it will do well.
The key will be in the way that Southend is marketed, in a way that emphasises its ease of use, quick transfer times to London centre and overall pricing. Easyjet's marketing of the place may help Aer Arann to get its routes better established in terms of putting Southend firmly on the map.
As regards the transfer of routes from STN I would guess that routes that are duplicated with Ryanair would be high on the list of priorities. However, If BFS gets a routes as is rumoured, I hope it is not a replacement of the STN route as this is a successful route and carried as many as 28k per month and would offer WW a leg up on the Belfast STN market, not sure EZY would want that either!!
Good luck to the new venture.
EI-BUD