Vickers
Its fairly obvious that if you live in Dublin or Edinburgh and you want to visit NY or Boston you'd fly there directly given the choice rather than Stewart, conversely if you live between BOS and NY (and millions of Americans do!!) and you can fly from Stewart to Dublin/Edinburgh without the hassle of JFK and to a lesser extent BOS then it makes complete sense to use it, it makes no difference, most will return to the US the same way so over a few weeks it will balance the flows.
Tech stopping in KEF from Bergen will depend on wind and loads, but the MAX shouldn't need to do so.
Most of the crews are new to Oceanic and will rightly take a cautious view on wx and fuel required until they get to know the routes, alternates and weather
The A321NeoLR can't come soon enough