while the "correct" pronunciation is kesshite, colloquially 決して is also said keshite. In any case, when sung, you don't necessarily hear the difference.
but I agree grammatically 消して doesn't seem to work.
what I'm asking for is if anyone knows any primary source (coming from the original songwriter or officially sanctioned by him) supporting 決して. 決して makes more sense, but it's only supported by secondary sources (those that have copied the text from elsewhere).