I don't want to get too OT, but on the specifics:

On the first point, I agree with you, but I assume that the word "stranglehold" was meant as voluntarily hyperbolic. At any rate, I agree with you it is excessive, but we are very much talking about arch-dominance here (over 50% is really very significant). One should also remember that the 55% excludes JV figures which are numerous for BA (AA, QR, JL, etc) so in practice, the dominance is even more than the figure suggests as JVs effectively remove competition by typically aligning prices and often sharing revenue (as is the case with the main ones here).

On the second point, BA is also by far the first operator at LCY and the second at LGW. This is noteworthy because even within a city, not all airports were created equal. and LHR and LCY are by far the two dominant business airport and they effectively exclude low cost flying (both long and short haul). This contrasts with, say, Paris where Easyjet for instance competes right in AF's garden in CDG 2 as well as ORY. This also incidentally underlines another interesting aspect of the BA situation. It is one of the very few cases I can think of in multi-airport cities where the main hub airline (BA) is actually based in the city's airport that is the one most passengers prefer to use. Now of course, I know some people hate Heathrow, prefer LGW etc, but the vast majority of people I know (and myself) prefer LHR to London's other largest airports (e.g. LGW and STN) - very short route to the city centre, nonstop tube to much of central London, and if one still hopes, let's assume Crossrail will still come in due time. By contrast, most people I know in Paris prefer to fly from ORY over CDG, most people in know in NYC prefer to fly from LGA over JFK or EWR, most people i know in Houston will choose HOU over IAH any day, in DC DCA over IAD, etc. In that sense, it is not that BA just dominates in two airports but not in others, but that it dominates in the two airports most of the population would ideally want to travel to/from, and that is in fact quite unusual.

From that point of view, I also disagree with your point on BA being "unique" in having a second local airline competing with BA. What do you call Easyjet or Norwegian or Level for AF? And I also disagree with the idea that BA is unique in having a second airline on most routes being unusual. Again, The same is very largely true of Paris or Rome, though indeed less of Frankfurt or Amsterdam which are less attractive destinations to serve for many foreign airlines.

Now don't take me wrong, I'm not saying that BA's dominance is worse than that of others, but I simply disagree that it is significantly less. To me, the BA and AF situations are roughly comparable despite having different specifics ("win some lose some" if you feel when you compare the two - BA has the more structured long haul competitor whilst AF has to deal with its competitors - U2, DY, and level - flying from the very same airport as it does itself), I consider ADP more biased towards AF than HAL is towards BA, but BA is the one which has its main dominance - and has managed to exclude low cost competitors - in the city's two "prime real estate" airports, etc). On balance, I just don't find their situation either uniquely favourable or uniquely unfavourable, just "mainstream" hub airline advantage