About whiskey plates.

It depends on whether there was a forfeiture action or the plates were impounded administratively. If the latter, your brother can file an affidavit containing necessary information and the regular plates should be returned.

I'm not a Minnesota lawyer, so I'm not familiar with "whiskey plates" or the rules about them.  You'd need to talk to a local attorney for more information about that.

You should be able to ask for a hearing about the plates.  Your state motor vehicle department is what lawyers call an administrative agency; there has to be a way to appeal any decision that MV might make along these lines.  In some states, the first appeal is within the agency itself.  Ultimately, there's usually a way to get a court to make the final decision.