ACK

In response to the original question of whether a retroactive corrective amendment can be made, it cannot be made under SCP. There are only a few reasons for corrective amendment via SCP, as discussed here, and I don't believe this situation satisfies any of these. https://www.irs.gov/retirement-plans/fixing-common-plan-mistakes-using-a-plan-amendment-for-correction-in-the-self-correction-program Otherwise, the corrective amendment requires a filing.

I don't know how the DB world works, but when I have this situation with a DC plan, if both spouses do want to be covered under the same plan, I still need to have a conversation with them about children, involvement, etc. Those things do matter because if there is a control group, then the plan is a single employer plan. If there is not a control group, then it is a multiple employer plan. So assuming both spouses want to be under one plan, I need to know whether I am indicating in the document that it is single employer or multiple employer and the 5500 is filed differently, depending on what type of plan it is.