It seems as though everything is reversed, i.e., you should have received the checks and paid the office 30% of your revenue and then you would have sent the office a 1099 for the payments.
Anyway, the checks are yours, and his cashing them without your power of attorney is forgery and fraud, but, if you're getting what you agreed to, it's probably a no-harm, no foul, unless the IRS thinks you earned more than you actually did, due to the weird 1099/EIN reporting.
As for the checks that he is holding hostage, that is pure criminal extortion, and you can call the police.
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It's pretty typical for law enforcement to avoid white collar criminal actions, because if they can get you to do discovery as part of a civil action, then the government doesn't have to spend any money investigating.
You could just tell the office manager that you'll sign the check and when he sets them in front of you, you just scoop them up and run. If you have a witness with you and he tries to stop you by force, then you have an entirely different police action: criminal assault.
Of course, I wouldn't want you to get hurt, either.
Re the mail issue, you could try the Postal Inspection Service. Nothing to lose, really.
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