Does the plan also have safe harbor? Match or nonelective? Discretionary match? Additional profit sharing? Does the bonus amount get included/excluded for different purposes? In other words, is this strictly a deferral problem, or are there also missed employer contributions for 15 years? The problem could get larger than it seems at first glance.
You might consider seeking ERISA counsel on this - it is possible, depending on the amount of money involved, the recommendation might be to ignore the issue and just correct moving forward. I'm making no recommendation, just tossing that out there for consideration.