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.
Candidates tout their lineage in every election. Certainly you know who the President's father is, and grandfather? if you can claim your family is decendant of a Mayflower family or some other historic figure (positive historic figure) wouldn't you want to share that fact? and really who isnt even a little bit curious enough as a youth or young adult to ask these questions of your family, especially the oldest alive relatives?
down south, we hear "fine Christian gentleman" all the time during election season for whatever it actually is supposed to mean or imply. candidates talk about heritage all the time. and if he doesnt update the bio's what will the next leftie/MSM attack look like?
will this help or hurt him? who knows, but people will judge him how he handles the attack.