I would be willing to bet that ammonia is more detrimental to walnut than acetone. And there are many top-shelf gunstock specialists and gunsmiths/stockers who have used acetone/lacquer thinner in very aggressive treatment of oil-soaked wood many, many times without damaging the wood. For all practical purposes, walnut, of any variety, has no naturally occuring oil in it, good or bad.

Bottom line, soak it 'til you get it.
JR

Be strong, be of good courage.
God bless America, long live the Republic.