The elephant in the room may well be competition authorities. BA, either direct or through one of their joint ventures, sit very heavily on the transatlantic market out of the UK. Having the authorities approve a take-over of Norwegian, supposedly including it's long-haul arm, is unlikely to take place without relinquishing other agreements.
I suppose it's exactly to get rid of the transatlantic, and growing Asia/South-America, competition out of Gatwick, that IAG are showing an interest in taking over Norwegian.
The matter is further complicated by the UK leaving the EU, and whilst Norway is not a member, it does abide by all the rules and subjects to the ECJ, in order to gain free access to the market.
Those contemplating the Norwegian aircraft being part of a deal, should remind themselves that the long-haul fleet, and the vast majority of the short-haul ditto, are leased.