You need to start by consulting with a maritime lawyer in your community. Special federal and state laws likely apply.
In general, the employer in your situation is smart to keep good records which accurately calculate commissions. Give the calculation to the employee. Encourage the employee to promptly report any dispute over figures
The internet postings may be defamatory or tortious interference with business. Making threats for money may be extortion.
It sounds like you might have counterclaims if you ever got sued. I sure would want to know though that you got your facts straight as well as corroborating witnesses and documents. Attorneys always want to know if there are any surprises or some compelling other side of the story