But what did the conversations look like behind closed doors? "If you don't work with us, I'll guarantee the judge gives you the maximum sentence" seems plausible.
They can't guarantee that. Even the judge can't guarantee it. The grouping rules for sentencing multiple charges are binding. It's not a situation where "usually judges are just cool and don't make you serve the sentence for every charge". Grouping is mostly not discretionary.
They can't guarantee it but they can say that they can guarantee it. They are allowed to use "trickery".
Which is why we guarantee criminal defendants legal counsel.
Which isn't worth much in some states, like Texas.