I was leaning towards the same conclusion. Either a non original badge or
a fork from another model, otherwise it's a dead end.

So, if we take the badge out of consideration, accept the fork as original and
the owner's claim about it being brown/ish then it could be a 1984 Granwinner
like you previously suggested.

Although this would mean that the frame is not a Tange Champion but
an ΗΜ-2 which would make it a bit "inferior".

Edit: Seller said that there was a "Tange Champion" sticker on the downtube
but it came out in pieces so he couldn't salvage it... Also, the seatstays have
small drilling holes like this one