[neatoman]

Obvious retcon is obvious, that's the first thing that comes to mind for me. There wasn't really any implication in the first three season that Ch'rell was impersonating anyone, they did give enough hints in season 4 for it to still feel organic but it's pretty clear it was an afterthought.

As for the concept and excecution, it's fine. He is effectively just Ch'rell with a different origin and slightly different goals, which is weird considering how they made clear he wasn't Ch'rell (that mystic bond between the Shredders might explain it away but that's a poorly explained concept itself), "Meet the new boss, same as the old boss". He might be OP and season 5 had a rather insane climax because of that (Dragon transformations? I mean, really?), but I'm not against the idea in theory.

As for the debatable human status? I don't care, Shredder was created to be starter villain to be killed off, he's not that deep or interesting in the first place. At this point most versions of Shredder have ended up with a questionable human status, who cares? It's not a big deal.