Only the Cohen evidence implicated Mr Trump in campaign finance violations. So ... pay the fine and we get "No collusion".

It doesn't work that way. Trump et al wish it did, but it doesn't. Accidental campaign finance law breaking is usually penalized by paying back the money, paying the fine and moving on. If you set out to do it knowing it is illegal and then go to great lengths to cover it up afterwards, that shows corrupt intent. That is a felony and people go to prison for it. The fact that Trump had his lawyer do it is is strong evidence that he had corrupt intent because any lawyer would have known it was illegal.