People are forgetting that he never fully developed the Justu and most certainly didn't develop it till hand-seals of all the things. Getting it to the point of hand-seals would mean that he fully developed the Edo Tensai and it would be used with the said hand-seals, since as far as the whole hand-seals and justsus go, only the completely developed jutsus have hand-seals. As the manga has never shown any incomplete justsu with hand-seals, it's safe to assume that Tobirama gave up on it fairly quickly.
Henceforth since he never completed it, he cannot undo the Justu he never completely developed. This isn't a plot-hole, but common sense.