All the Sun had to prove was that what they'd printed was substantially true. Amber Heard's behaviour wasn't in question and this wasn't what the court was being asked to decide. As soon as he admitted in court that he beat her, the result of that case was a foregone conclusion.
Looking at his claim and her counter claim I strongly suspect both are telling the truth. He's now on record as a wife beater, having publicly admitted the fact; she's admitted on a recording to having got physical with him. It's going to be difficult for either to prove they are the innocent party given that ugly box of tricks he keeps persisting in opening in court.
Her counterclaim, that he's smeared her reputation, stalked her online and harassed her using the legal system for years, seems pretty much unassailable to me. There's a demonstrable track-record of it.
As to the abuse, having seen that repulsive exchange of texts from Depp, addiction issues aside those are not the words of a nice, kind, benign sort of bloke. As for the constant protestations from his deluded fans that 'he was just letting off steam'; 'we've all said that about people we're angry with' - er, no. I've never entertained such hideous fantasies and ill-will toward any person. They are disturbing, disgusting, misogynistic, and only add plausibility to her claim about the sexual assault.
Sounds as though both had awful backgrounds which seriously messed them up. It was their misfortune to have found each other, but the one clinging on like a dog with a bone and who won't let go, is Depp. His career's tanked so he has nothing to lose except, apparently, taking her down with him.