A nice set of observations Nick!
The changes in magnitude are really quite spectacular. I have to confess I can’t immediately explain these large changes in luminosity. Of course, accretion disks are very dynamical and complex processes occur within them. Now in the case of variable quasars, it has been recently suggested that if the accretion disk is not homogeneous and there are large local temperature changes, then this can result in the light being somewhat bluer when the qusar brightens and one would expect spectral variablility. I am not certain whether this mechanism could explain the osbervations in a binary system, but it would certainly be my first course of investigation!
The paper on large temperature fluctations in Quasar disks can be found here arXiv:1401.1211