Don't forget. 6E used to run the Flying Returns backend. they know more loyalty programs s/w and airline resv. systems than anyone else in the indian market
True. And besides, since this whole thing is largely conjecture anyway, it may not even be necessary for Indigo to start an FFP of their own or stray too far from their current model.
In recent times, many airlines have been venturing down many new, different paths. BA could simply place their code on a specific set of 6E flights that align best with the London operations and give EC members a thimbeful of Avios. It also leads to very limited interlining and baggage transfers for Indigo.
This is perhaps unlikely, but not too far-fetched I don't think.
I still don't see any great benefit for BA though.