Is it permissible to reveal part of a woman's hair in front of non-mahram men? Is there a consensus on its impermissibility? And is there a valid difference of opinion on this issue that does not warrant censure?

There is no disagreement among scholars regarding the obligation for a woman to cover her hair from non-mahram men, and the impermissibility for men to look at it. Imam Abu Bakr Al-Jassas and the Fiqh Encyclopedia have both mentioned the consensus of jurists on this matter. There are many proofs, including Allah's saying: (they should draw over themselves some of their outer garments).