Actually, my experience is that Multicam in Premiere Pro is the devilish problem.
Coming to the end of a sizeable multicam project, Premiere began unable to handle my project. I tried everything that you did – this was on PC, and I even moved it to my Mac. In the end, I couldn’t afford any more downtime. I moved the project to Avid.
I never found what exactly was causing the problem – was it audio, video? Adobe was not able to help. I hope you are able to figure it out – but I don”t think MXF files are the problem.
Good luck