But what could Kate Moss say that's so massively damaging to Amber? We already know they had a somewhat volatile relationship with Johnny smashing up at least one hotel room. That doesn't mean he hit her (Kate) of course or was controlling, jealous, verbally abusive etc.
One could still argue that destroying property like that is abusive and intimidating in and of itself, so unless she's coming on the stand to say it was both of them having the time of their lives then that reflects badly on Johnny. And she will certainly be questioned about the incident.
She can absolutely debunk the rumour that Johnny pushed her down some stairs, but that doesn't alter the fact that Amber was aware of the rumour and that factored into the situation with Whitney, as she has testified.
She can say how lovely he was and that's obviously a positive for Johnny, but it doesn't cancel out Ellen Barkin's testimony or the fact that his jealousy has been mentioned by other witnesses. All she can really do is say what her experiences were, which as we know don't necessarily reflect how he may have behaved in other relationships.
You could argue that neither does Ellen Barkin's testimony say anything about other relationships or use Kate to discredit her as a 'woman scorned', but that will ultimately be up to the jury to determine. If it's a case of determining whether he had the capacity to be abusive, I would say that evidence from one partner (or two including Amber) is enough to suggest that he could be.