Honestly I can't think of anything less transparent than a joint Russian-Egyptian investigation
I have enough faith in Russian investigators. You can say what you want about Russians, but they have some excellent aviators, engineers, and experts. I do think they will be able to discover what happened. There should be enough clues now, wreckage, recorders, radar data, mx records; further, there will be quite a bit of public pressure in Russia to figure this out.