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I don't think that it is something new. I seem to remember that he also have a third eye when he went berserk in the earlier chapters. that time when he first encounter David when he is recuing Leo. It may be what he inherited from his mother? Probably? From what we have seen, everything that is related to Martial Arts is from his father while the Mentalism Arts are from his mother? At least, that is what I think.
Now that you mentioned it, I think this is the reason why Uno disgust Shin too much. As for the one who take over Shin during the lab incident, it was not Uno, it is the SEED taking over Shin's body. And it said before, in Uno and David's conversation, that Uno devoured that demon right after Shin past the so-called test or something.