If you've been told by BA that they seem to think the very thing that you want to hear, I'd take that information with a pinch of salt. It could just be a technique to get you off the phone. If FTers don't have the definitive answer on a question as obscure and transient as this, the chances are that a BA call centre agent who seems to think X doesn't actually have the answer either.

I can't immediately think of any situation in which a flight marketed by a non-partner but operated by a oneworld airline has ever qualified to earn Avios / TPs.

And the situation of a non-oneworld marketing airline for a flight operated by a oneworld airline is reasonably common. Yet I don't think there have been reports of routinely successful double-dipping in these circumstances. Airlines are usually pretty good at crediting no more than one lot of miles (even if sometimes even one lot of miles is a stretch, and even if the destination of those miles is sometimes difficult to predict in advance).