If they do brainwashed!tadashi for the sequel, wouldn't it be heartbreaking if the villain made "Tadashi" say things to Hiro that the true Tadashi would never say; or would that be too cliché? How you could do this without it coming off too forced?
The idea itself of the villain telling Tadashi to say these things does seem a bit cliche, but perhaps it’s what he says or the outcome of how Hiro takes it that would make it seem less forced.
With Hiro being the genius that he is as well as the one person that knows Tadashi better than anyone, he has to know what Tadashi is saying is not him at all. He probably wouldn’t be able to come to a direct conclusion of why Tadashi is saying what he’s saying, but I think he’d come to the possibility of him being brainwashed. The only thing him and the rest of the team would have to do is confirm it.
I imagine Hiro would ask Baymax to scan Tadashi, but I doubt brainwashing would come up as a diagnosis (as it’s not a medical term), but I bet there might be some brainwave level differences. And Baymax would probably still have previous scans from Tadashi before the fire and Hiro would compare them.
I think the others would jump in with noticing things from their encounter with Tadashi and the villain (like if the villain had a distinct signal or phrase that made Tadashi say those things) further putting the puzzle together.