But the T&C also say that you can only transfer to a FF account in your own name - whereas we all know that you can in practice transfer to ANY FF account if you do it online.

So in practice, whether the bonus pays per sending Amex MR account or per receiving BAEC account going to come down to how the IT folks implement it. It seems that there may be at least an outside chance that it will go by BAEC account; if the IT guy looking at the rules it wouldn't make any difference, because the T&C say that the result is the same either way.

I have 6 different BAEC accounts in my household account. I made 6 separate transfers, instead of a single larger transfer. It makes no differernce to me either way - it's all upside. Maybe I get 5k, maybe I get 30k bonus, but I have nothing to lose in mking 6 transfers instead of one.