First, Google has been released from the gag order on this NSL, so as I understand it Google could publish the letter now with no penalty.
Second, the constitutionality of these gag orders has been seriously challenged, so if Google was willing a fight they could probably publish any NSL they wished. Gag orders have historically been the purview of courts, and judges take a dim view of other people doing their job for them.
Third, it's safe to assume Google tracks revisions to their pages, so yes, they would soon know who made the 'mistake'. Also, a letter like this should be shared with extremely few people within the company, so it shouldn't be hard to follow the chain until suspicious activity is found. Punishment for this sort of mishandling would be limited to a fine, however, so the FBI would go after Google's deep pockets rather than try to pin the crime on an individual. The employee should be safe from criminal charges, though not, presumably, from Google discipline.