(I worked in MN child support until last year.)
If the employer considers her full time, then CSED will consider her full time for child support purposes and will not attempt to impute her potential income or deem her "willfully underemployed".
When the statute refers to "willful underemployment", they are mostly referring to, for example, a person trained and skilled as a welder (capable of working $40+/hr) working as a clerk in a shop to avoid paying child support based on what he or she is fully capable of earning.
Your wife is working in an industry where 72 hours is considered full time. The industry typically schedules that way intentionally, because then they don't have to pay overtime to the employee when they are called in to pick up a shift.
You aren't going to win arguing willful underemployment in MN. The magistrate is not going to see it the same way you do.
