As long as he didn't sign a contract with Barcelona they risk nothing, what could happen for Barcelona is that they could be sanctioned for offering a contract to a player under contract without his club's approval. I don't what they risk and I don't know if it's demonstrable.
That's the crux of this whole affair. If Atletico can prove their claims, their case might have legs.
As for La decision it doesn't really demonstrate a particular amount of stupidity and definitely not point to someone who would leak something important, particularly when he kept his decision private until the last minute.
Yeah, stupidity isn't right. As i said, it's more of a stunning lack of awereness - or a deliberate disinterest in public perception and an unabashed "look at me!!!" move - though he most definitely did not keep it private until the last minute given that he'd already made his decision for days and it had already been leaked to the press
Still, this is different. Atleti's basing their case on him and barcelona reaching an agreement before the buyout clause went down, and him communicating he would be leaving at the time he did reinforces their case. Atleti will still need to provide further evidence of it most likely, doubt this would be enough, but given the situation it was a pretty dumb move. His agents and advisors did a pretty poor job there...