I’m not a fan of walnut risers, not when other woods are cheap and a better choice, ie. Bolivian rosewood.
That being said, what I would have done is shorten the riser as was your plan. Get rid of the bad spot. Then make a power wedge out of something tough. I do this by making about a .070 parallel and tapering the last 2 inches on each end to nothing. Whatever you want your finished riser length to be, make the PW an inch longer, as the fades are so much finer. For example if your plans call for an 18” riser but all you have is 16” or 17” to work with, then make your power wedge 19”. It would let you turn that shorter riser into an 18” riser then you could have went with the 58” bow. Make sense? It also adds a lot of strength to a marginally strong riser wood.