Must be proven first? Define "first". It certainly needn't be proven to file the suit. I'm living proof of that.
"BTW, when is truth NOT a defense to a charge of libel?"
When you can't afford a lawyer, and can't interest a public interest outfit to represent you because they already have enough "Does" on the case, and can't afford to travel halfway across the country for hearing after hearing after hearing to try to represent yourself.
You have no choice. If you don't defend against the suit, you lose. If you can't afford an attorney, you probably lose.
There's no solution to that problem on the horizon. I'm sorry you were on the receiving end.