I agree that it is unlikely that 6198 should have been used.  Assuming that is the case, the prior tax returns should be amended.

Can you clarify what you mean by NOL?  If they used 6198, that would presumably not have had a NOL (net negative income across the entire tax return) because the 6198 suspends the loss.  If you amend to undo the 6198, that could create an NOL.

If there is a NOL, you just follow the usual rules.  Depending on which tax years are involved, it is carried back 3-5 years.