Unfortunately, it's like peeling an onion.

The compliance issues are weekly, monthly, quarterly, and/or annual, for US payroll. If you are supposed to catch them up fully, you have due diligence for the calendar year, not the tax year. Technically, if that's not what you're hired to do, then you work with blinders, but I would make sure to note this limited scope.

Otherwise, the settlement should include a look at/for benefits, such as retirement matches, health premiums and policy notifications, etc. Ugh.

Let us know what you find.