Thank you for trusting your question to JA today. I am a licensed attorney with over a decade of law practice and over 20 years of experience in the legal field. I’m happy to be of assistance.
You have a few legal problems here that would make this very difficult to resolve. First, a verbal promise of confidentiality is nearly impossible to enforce. There is no automatic confidentiality attached to those things we say at work, even when part of an investigation. While I understand that it is too late now, it would have been better to have something in writing. Then you'd be able to sue for breach of that agreement. Here though, you don't have an agreement. You have a verbal promise, which can be contractual, but which only you and the other party know the terms...and if your statement to a jury isn't more convincing, you'd lose that argument (as you bear the burden of proving there was an agreement and they breached it).
And you have to be able to prove that to have anything here, because no other law would make this situation illegal in any way. You don't have to be forewarned of a deposition. You can simply refuse to speak during it if you don't want to do it. They don't have to give you any particular notice it is coming. Comments about what you said and what happened are not defamation of character, because for there to be defamation the statements have to be false. A true statement can never be defamation, so truthfully repeating your statements, no matter how damaging, is not illegal.
You need to try and memorialize the confidentiality agreement in writing in some way. I would send them a communication, through a letter or email, stating what you were told were the terms of confidentiality and that you feel they are not following those terms. That gets what you believed in writing. If they respond, it is very possible that they'll respond in a way that adopts some, if not all, of the terms you state. THEN you have a written set of terms with some form of agreement from them that those are the accurate terms. That would make any sort of breach of confidentiality claim very strong.
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