My tune has not shifted, accident investigations are never perfect, data is never complete, reconstructions are never perfect, if there's anything that I've learned in the over two thousand diving fatalities that I've analyzed, it's that the simplest explanation is most often the most accurate.
The simplest explanation here is that the Instructor screwed the pooch. What she needed to do was grab the victim's tank valve and establish positive buoyancy, she failed to even attempt to do that and seems to have gone astray and become involved in some sort of completely unnecessary struggle with the victim. Why she did this is a question of interest, but the exact reason has little bearing on her failure to secure a strong grip on the victim's tank valve and establish positive buoyancy.
She will, doubtless, have rationalizations as to why she was unable to accomplish these simple tasks, but the fact remains ... she came back up alone.