Thanks! That does help clarify things, and I probably am overcomplicating them!
FWIW, the Aer Lingus component (as a qualifying flight for the tier upgrade) is new, even though both are IAG. I had Aer Lingus flights considered invalid for tier purposes as recently as October 2023. So while AA has been a closer OneWorld partner all these years, it is helpful for me to hear from others with personal experience that any AA flights I may book will count toward tier status - especially when some flights are better scheduling- or price-wise than BA. Otherwise I generally prefer to fly BA metal, so again, newish territory for me.