The apology is off the table. Union view is that any apology will be seen as admission of guilt and will be used against me.
Their view is there is zero evidence until I do. Claim is based on hearsay and this is why management want it done informally.
You're still not being clear here - what is the actual issue? What are they claiming happened?
1. Conversation happened.
2. I expressed an OPINION on a factual happening, nothing I stated was untrue.
You're not being clear here - stating some facts or airing an opinion isn't necessarily a disciplinary matter - was there something specific about that opinion that might make it one? Or are the making some additional claim re: things you didn't actually say?
If you didn't do anything wrong then you have nothing to apologise for and your union rep is correct.
If you really have company blackmail level knowledge the smart move is to use it to stop the process, not to try to change the outcome.
No, that would be dumb too. The smart move would be to not get involved in any blackmail in the first place.