It certainly will be interesting to see what happens regrading the BMI merge into BA/IAG.
Its hard to ignore looking at what has happened in Glasgow recently, whereby the withdrawl of BMI led BA to take all the market, and use the 767 each weekday morning over the winter, but would there be a similar demand in Belfast??? It'll be interesting to see if similar things happen at Edinburgh, since I think some EDI slots are part of the sacrifice BA need to make to let the takeover go through.
My 2 cents (and its just an opinion) is that BA will continue to use Aer Lingus as a codeshare from Aldergrove.. with slots at a premium, and the openly admitted desire to expand more in Long Haul out of Heathrow, why would BA ruin an agreement that works to their advantage? Sure, he could pull out of the codeshare and take all the business, but is Walsh going to want to part with more slots that he would rather use elsewhere?
Incidently, the based EI A319, is it to operate mainly international flights, or the London run?
BMI manage to operate upto half a dozen flights a weekday, mostly on the 319s, occasionally the 320s. Even if BA reduce the number of flights a little, and utilise the likes of theA320 and A321 more often, there should still be enough capacity available with just that.
Regarding any preference to BFS over BHD, and performance issues, there are no real issues for an A321 operating into Belfast City. Thomas cook can operate the higher capacity A321 direct to Spain so the lower capacity BA 321 (188 seats or so) to Heathrow should be easier, though they do go through brakes a lot quicker at Belfast
BHD2BFS- what you said about keeping it all under one roof, and having both operate from Aldergrove will suit the passenger that, as you said, needs a quick transfer to another flight to Heathrow, however i'm not convinced or aware of any financial benefits of having both operate together against two airports, as they're both seperate companies, and both have their own prices and arrangements with the airports, handling agents etc. regardless of what airport they use. If, for example they were to merge, or self employ more staff, rather than outsource, then it would be a massive benefit to have all under one roof. You could also argue that having access to Heathrow and BA connections from both airports could appeal to a wider market, and keep everyone happy.
Lastly, and probably a long shot thought, but if Willie Walsh wants to return properly to NI, could we see Cityflyer operating from London City, or the Gatwick 734s? (the latter could hurt the Flybe codeshare though). It might be a limited market, but it seems to do well to other cities on mainland UK.