You could also use the subduction of the ridge ca. 250 Mya as the driver for the breakup. I'm guessing this is more-or-less the scenario you were envisioning, with a hinge-type rotation between the separating landmasses. In essence the, eastern portion of the "southern plate" is subducting, while the western portion is diverging. After initial break-up, the rift would spread further east, with a piece of the southern plate breaking off. I guess the end result is largely the same, but I thought it was worth mentioning.

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After the separating piece collides with the northern continent, it would be expected for the subduction zone to "jump over", and subduction to continue until the mid-oceanic ridge has been subducted. What happens beyond this point gets a bit complicated.

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The subduction might cease (depending on the relative montions of the plates), since slab-pull would no longer be a factor. However, unless the two continents merged into a single plate (with a mass of oceanic crust in the middle), a new rift would have to form. This would likely form over continental crust. Like you mentioned, another possibility is that the subduction continues, in which case the continents could return to the initial hinge-type movement, eventually colliding. It's also possible that another portion of the southern continent would break off, repeating the cycle.