Are Deferral Failures Discrete or Collective?

A plan failed to allow deferrals for some participant beginning in 2015 and continuing to 2017.  On plan correction, how do we interpret Rev. Proc. 15-28, which allows correction using a 25% QNEC if within the SCP time limit for significant errors (last day of the 2nd plan year).

It is clear that for 2015, the 50% QNEC correction applies, since 2018 is beyond the 2-year deadline.  The question is for years 2016 and 2017.  Are each of the missed deferrals treated as discrete and therefore eligible for the 25% safe harbor correction or are all missed deferrals considered part of a continuous whole and all subject to the 50% QNEC correction?

Please direct me to any citations to support your answer.

Collective. 

Check out this thread and see my post that links to the q/a: