Ned Stuart definitely could have been played by someone else--watching it recently I wondered if the police would buy that he was just a random mirror image, as I seem to remember jeff clark was under suspicion of certain crimes when he departed. They could even have put him in a wig, makeup, and funky costume, ala what was done to create Sandor and Magda in 1897; he didn't have to look exactly the same.
There was something satisfying in RD's Tate character--[spoiler]he played well the dejected, violated, discarded 'boy' of Petofi. [/spoiler]