Correcting 410(b) in a 401(k) Plan

The only participants previously covered in a 401(k) plan were owner and spouse, neither of whom make a deferral contribution for the year in question.  The plan is not safe harbor.  It turns out that because of a recently discovered ASG situation, 3 NHCEs of a member of the ASG have to be covered for 410(b) purposes.  Under the 11(g) regs, coverage should be corrected by retroactive amendment and a QNEC equal to the ADR of the NHCE group.  How do you determine the QNEC when there's no ADR for either HCEs or NHCEs for the year? 

There's also a cash balance plan affected.  These NHCEs will be retroactively covered, provided with a meaningful benefit under (a)(26) and receive profit sharing contributions in order to satisfy (a)(4) on a combined basis.  But how to correct the 401(k) component? 

Not sure, but when you use prior year testing you assume 3% ADP for NHCEs, right? So wouldn't that be a reasonable level here?

For CBP, owner and spouse are 40% (2/5), so you don't HAVE to include those NHCEs unless they should have been included under the terms of the plan. 

Is this a last year situation or a multiple year situation? 

These 3 NHCEs are being provided meaningful benefits so that the CBP will satisfy (a)(26).  There were more HCEs and NHCEs at the related company.  The amendment will only name these employees as eligible, without extending participation to all employees of the related group, to the extent needed for 410(b) and (a)(26).

This is a multiple year situation, going back to 2019.  With the interim IRS guidance on broadening self-correction options, I believe that if we correct through a retroactive amendment that would have satisfied the 11(g) rules, then the plan can self-correct.