Yes, extortion is a criminal act but "coercion" is a type of extortion. In this case perhaps I should have just stuck with coercion. And you can certainly get civil damages for financial coercion. Regarding civil damages for a criminal act, you can sue as a victim of a crime and you can get damages. Just google "Civil action for damages for criminal act" and click any of the links on the first page for a variety of state code on the matter.
None of this applies to you because they are requiring you to comply with
the law. There is no crime being committed by MtGox and nor are they committing any tort. Asking you to identify yourself in accordance with AML/KYC requirements is
not coercion or extortion. Nonetheless, I really hope that you do make a complaint to the A-G and file a civil action. You'll give the lawyers some free entertainment and likely end up with your financial accounts being investigated for possible tax evasion.
ID or no ID, there is absolutely no good reason an untouched $100 cannot be refunded to the known account from which it originated. This should be the default action taken if the customer fails to provide ID upon request.
This may or may not be true. Many jurisdictions have a minimum level of identity verification which must be applied to all accounts no matter how small the balance/transactions and funds can and will be held indefinitely if the customer does not satisfactorily identify on request. Certain types of transactions trigger ongoing and enhanced due diligence requirements and providers can ask you to re-verify at any time.
Dwolla has put some very onerous KYC requirements on merchants with little or no notice. In addition to their ToS now stating that neither merchants nor users will use Dwolla in association with any virtual currency without written consent (MtGox is still waiting on written consent and I suspect that you haven't sought it), starting 17 April Dwolla imposed the following conditions on users sending money to the accounts of merchants who've had reversals.
Have a bank deposit 30 days old
Enable a DWOLLA hub page
Verify SSN