Not crazy! This idea actually reminds me of a paper that was published last year, which found evidence of an episode of passive margin collapse on the western margin of India. The cause that the paper proposes is compression brought about by India's rotation at that time provided the impetus for sunduction initiation in the form of continental crust overthrusting oceanic lithosphere. So, what we need to look for in your project is a moment where the southern margin of green is undergoing compression. At some point the distance between green and the southern supercontinent needs to get smaller. This is made possible by the fact that it is moving due to slab pull, and is smaller than the southern supercontinent (less drag).

Regarding the unresolved mechanism for initial breakup, don't be afraid to invoke bottom up tectonics. It might actually make this second phase more plausible since the northern and southern supercontinents would only be drifting away from each other due to ridge push (quite slowly). This should ensure compression on green's southern margin once it splits off the northern supercontinent.