Out of curiosity why are we assuming CO and ERW?
Delta for instance have stated their transatlantic business strategy vies-a-vie acquired 757-200s, is to use them on European routes which have little or no American scheduled service.
More likly that Delta would do LBIA-JFK then CO doing the Newark run imo.
I would assume CO because it fits their strategy more, Delta’s policy of 757s is less mature and they have been in negotiations with a lot of airports, but with Delta there is a lot of talk, and a lack of action sometimes. Unlike CO.