I seriously doubt he is Clow. I would be more likely to believe Xing Huo as Clow than Watanuki. I'm not ruling out that I am wrong, but he can't be SO MANY things at once, it'd be overkill. For him to be Fei is possible, but just seems strange. Watanuki was created in the long run to be used against him was he not?

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He can't possibly be Clow. I mean, he is a time-clone of a boy, who is a son of his own clone. He can't be a copy of his copy's ancestor at the same time! Or I hope so...

I have follow Clamp since their were Doujins makers, those girls like to create this kind of discussion, but they follow a simple line of events, and they dont hesitate in put something absurd in the plot and then much later justify it. I believe also he is a previuos version of Clow or has to do something with him. Because Clow has strong links with Sakura and Syaoran in Card captors world, in the serie Clow was like I have to make them Happy in this world, most likely Sakura wasnt mean to be Clow sucessor if no looks like the cards was create for her from the begining including Yukito and Kerberos but that also is only my thoughts(how why the book was on sakura house until I remember never was explained), the girls of clamp likes drama no matters if it isnt make sense, even her romantic comedies are heavy like chobits.