I just realized I could've applied the Russian Roulette optimization to the new photon scattering method as well. The fundamental difference between the literature photon scattering method (from Per Christensen's paper) and the Georgia Tech method is the photon that is being stored. The former stores the incoming photon, while the latter stores the outgoing photon. This is why the latter allows us to compute the direct illumination as well as the indirect illumination.
There are improvements. First is performance. I am able to cast more initial photons and end up with fewer photons in total. The results also have more subtle and pleasant diffuse reflections compared to before. (Notice the nice blue diffuse reflection below.)
Here's the result. 120 initial photons, 2 bounces, ~200 photons total.