Does Sebastian ever refer to Ciel as "Ciel" in the original Japanese? I don't know if this is the same in the Japanese, but as far as I can tell, in the English version, Sebastian has never referred to Ciel as "Ciel". Only as "Master Phantomhive", "My Master" or "Young Master". If it's the same in the Japanese, it might add further credence to the 2CT, as Sebastian is not allowed to lie, and if the theory is correct our!Ciel is not really named "Ciel".
Throughout the series, there’s only one scene where Sebastian called our!Ciel “Ciel” and that’s this scene in ch5:
It’s Sebastian’s monologue, and as you can see, he clearly states the master of the house is “Earl CIEL Phantomhive”.
But aside from this scene, he has never called his master by his name, he only calls him “bocchan” (young master). Even when Sieglinde asked Ciel’s name, Sebastian didn’t answer that question himself, but let Ciel answer it.
But when asked about Ciel’s age, Sebastian had no problem answering it instead of his master.
Anyway, aside from that one exception in one of the earlier chapters, Sebastian basically never calls our!Ciel “Ciel”! :D