I guess I just don't see it the way you do. I just can't think one has to research grandparents as part of a career in public service, and I don't think Allen looks "stupid" for not knowing this fact. Offhand, I can't think of any information I know about any public figure's grandparents, other than Bill Clinton's, of course, since he played his dysfunctional childhood navel gazing card when it suited him.
As for campaign biographies being rewritten-- if your grandparent's religion is a key feature of your campaign biography, then you should rewrite it anyway! Did his biography affirmatively state that his grandfather was Christian? Or was it just not mentioned? I would hope it was the latter. To say he should rewrite his campaign biography to disclose a Jewish heritage would make me very uncomfortable, as I just don't see the relevance of his grandfather's faith.
As for precious campaign time down the drain, again I disagree; I think most voters are responding favorably to Allen on this exchange.