Backbiting

If he was out of his mind or was ignorant, why would you want to expose him?

Let her cool down for few months and then try again.

As long as they stop, this is in sha Allah sufficient.

That is not Backbiting, it is a testimony.

NO it doesnt. You have to seek forgiveness from Allah from the committed sin.

Kafirs don’t take muslim’s good deeds. However the muslim is losing these deeds while backbiting a non muslim or talking  about them for no legitimate reason as such gossiping is not appropriate for Muslims.

Both of them are sinful. One must not defend himself by backbiting others before verifying the truth or facing that individual. You can say the truth without backbiting him.

No, this doesn’t cancel it. You need to repent and seek that person’s forgiveness if it wouldn’t cause any further damage or more harm. 

No, this is not permissible unless there is a legitimate reason for such an act.