Frankly, I think that this is a lot of song-and-dance over something utterly trivial. Clearly from what the experts here have told you; you had the wrong idea. These codeshares are very complicated for normal people as they very and change so much. The boys have done a super listing which explains everything in the Wiki. They do this out of the goodness of their hearts and I, for one, am very grateful.

I am unsure why having priority boarding was such an affair of state for you on a flight to Dublin which starts descending almost as soon as it reaches cruising altitude. I’m also unsure why being denied this caused you so much indignation and why you should be embarrassed I cannot tell. I’d have just said sorry, I thought that it was allowed and left it.

One thing that I have learned here is that the codeshare means precious little except earning your Tier Points and Avios. They may lip service but nothing more.