Ah, nothing like the rabbithole of plate tectonics, where you can always make things more complicated!

It's always tough to suss things out without playing around on GPlates myself, but I think my instinct here would be to have the southern A/B margin as more or less a transform boundary, and have it running northwest to southeast, somewhat parallel to the direction of microcontinent B. The other option might be to have something like what you have at 150 Ma, with the subductive margin extending southwards beyond the continent; this is perfectly normal in plate tectonics as far as I know, though I might expect the subduction zone to keep moving to the west as there's no continental margin to stop it; this is more or less what happened with the Scotia Plate, and I think maybe the Greater and Lesser Antilles.

But these are all just guesses and I'm by no means an expert- getting a realistic tectonic history is a cycle of constant frustration for me, even though I find myself steadily happier with my results as I go on...