smells centralization...

Nope. Just administration of the guarantee (the backing). Everything else is decentralized. The token still trades without interference from a central authority. But unlike bitcoin which has nothing backing it, Tether will have a pot of cash stashed away (with an administrator - hopefully trusted....)
That's centralisation. Requires trusting a human(s). Admin runs away with the reserves and the tokens are worthless.
How is fiat reserve proof verified in a real-time, automated fashion in a way that isnt manipulatable? Humans can intervene with the reserve indicators, no?
Yeah but there's nothing wrong with it. If you trust them, they could be fine for whatever your business might be. As a business you always have a certain level of risk when dealing with other businesses, but at least you have legal recourse.