So just to be clear: The annulment (in the show) wouldn’t work for Rhaegar because he already has children with Elia? But the plot to make Margaery Robert’s queen makes sense because the twincest would be revealed before any actualy attempts at marriage between them? Margaery was only to become Robert’s mistress until such time as they revealed the twincest? (I’ve seen these situations compared, usually using the latter plot to try and explain the shows annulment idea). Thanks.

It’s always possible that Westerosi law differs from OTL Western European law, but yes, it would be unlikely that an annulment (as opposed to a divorce) would be allowed in a case where there are children, especially a male child.

In the case of Margaery, the plot only makes sense if Cersei’s children can be disinherited due to revelations of incest and adultery. As for “until such time,” I think it was planned to be very quick in succession: Robert gets infatuated, wants to get rid of Cersei, Renly steps forward with the awful truth, Robert is enraged and humiliated, hey presto here’s the lovely Margaery in his bed to console his ego and produce legitimate male heirs, what a lovely wedding/war…