Thanks for the feedback, all. I spoke to BA customer support who checked the return flight (HND > LHR) and saw that seats 30CDF (yes, CDF, not DEF) were reserved - her guess was that three Gold members had been allocated those seats, and that it was not related to disabilities. None were infants. She said that once the 'airport team' began reviewing seating requests like ours for a bassinet seat, that these passengers would likely be moved for us, unless they did need those seats for accessibility reasons.

I have spoken to two people on the phone and three on X, and have had multiple different conflicting explanations given, though. Hers was the most recent and the most coherent so I'll guess that will have to suffice for now.