Yes, splitting the rca is perfectly safe, and the very low output impedance of the cdp will mean that any sonic effects of having two loads and two cables attached to each of the rca outputs should be negligible.
Ground loop issues might still be a possibility, along the lines of what Dave suggested. But if need be those could be addressed with cheater plugs, or connecting to different ac outlets, or isolation transformers or other measures that are commonly used to address ground loop problems.
Re your last question, if my theory is correct, then yes the buzzing would remain if you continued to use the adapter cable. It is conceivable, though, that it might be avoided with a specially constructed cable that left pin 3 unconnected.
Good luck!
-- Al
