Correction of employee erroneously excluded for 2015

[Carol V. Calhoun]

I've got a plan that erroneously excluded a total of three employees in 2014 and one in 2015. It looks like the employer can use the new corrections procedure in Rev. Proc. 2015-28 (requiring only a 25% QNEC instead of a 50% QNEC to make up for the lost deferral opportunity). For 2014, it's fairly clear how this is to be done. However, I've got questions with respect to 2015:

1. Is there a missed deferral opportunity at all for 2015? The Rev. Proc. says you've got to give the employee a notice saying that he or she can make up the missed deferrals, subject to the 402(g) limits. Since we're only halfway through the year, the employee should be able to defer the full 402(g) amount in the second half of the year. At the same time, the procedures are different if the error is discovered within 3 months than if it is discovered thereafter, so presumably some correction is required if the error goes on for more than 3 months but still within the same year?
2. If the employer has to make a QNEC for 2015, how is it calculated? The QNEC is based on the ADP of the HCE or NHCE group. But presumably, the ADP can't be calculated until after the end of 2015. Does the employer have to defer the contribution until after 2015 ends, or is there some way to determine the amount of the QNEC before that time?
3. The notice to the employee must include "A statement that appropriate amounts have begun to be deducted from compensation and contributed to the plan (or that appropriate deductions and contributions will begin shortly)." What does this mean in the context of employee elective deferrals? Presumably, no amounts should be deducted from compensation until and unless the employee makes a deferral election.