“That now lets them probe if he told Trump, which is surely part of the plea deal for him to say “yes I told Trump too”. Which means that this liar is accusing Trump of perjury.”
Sounds plausible, but Cohen supposedly taped Trump’s conversations with him. Thus, if this is not on a tape, his testimony will be essentially meaningless. Trump can just stick to his guns, and state that Cohen is either misremembering or is lying. He can also point to the fact that Cohen has changed his stories, and admitted perjury, thus has admitted to being a liar.
The thing that I note is that we're hearing leaks now that Mueller is probing Rodger Stone about his private discussions with Trump via telephone. Here's a plausible scenario: The FBI has tapes of all of those calls and Mueller has heard them but he doesn't want to say that publicly because it's one thing to be monitoring Papadopolous based on BS manufactured intel, it's another thing to extend that to conversations between the candidate and people not on the campaign. And I think at least some of those conversations happened prior to the FISA warrants so having them is illegal. It's impossible to release them because that confirms that Trump was indeed personally being wiretapped by Obama. That's hard to spin as legit. But Mueller thinks he has something good in those calls, something that he can use so he's squeezing Stone to get him to say it in testimony and eliminate the need to produce those tapes.