I don't know the specifics of MN law, but per your statement that the custodial parent's income is not considered for the purposes of calcuating support, this may or may not forclose the issue.
There MAY be MN caselaw that mandates both parents work if they are reasonably able to do so, however, I don't have time to research the issue for you.
As far as this child support being actually spousal support, that argument has some merit in a situation where the custodial parent refuses to work and has no other sources of income. However, in order have the court declare some portion of your payment as spousal support, a support modification action must be pending, and I don't know if you have grounds for this currently.
Would you win such an argument? Without an attorney, I'd say your odds are effectively zero -- with an attorney, well, who knows? There may already be caselaw on the issue in MN. You'll need to reaserch it yourself or ask a local attorney.
