It occurs to me that the co-cospirators have thoughtfully written down their advice and contributions in various media - text, emails, etc. - and those would be admissible. I suppose the defense could try to refute it by calling the co-conspirators, but then that is not the prosecution forcing them to plead the fifth and generate inferrences. The defence would have to be careful what they ask to avoid straying into fifth territory, as would the prosecutors. I assume there’s a legal principal that once the co-consirators answer certain questions for the prosecutors, they cannot refuse to provide more detail on cross-examination since they’ve “opened the door” to that line of questions.
A commetator on MSNBC made an interesting point - legally, if Bob says to Charles that Andy said “X” then that would not be admissible in Andy’s trial, it’s hearsay - even if it is a written text or recording, not just testimony. Howver, if Bob is a co-conspirator, even an unindicted one, for the charges Andy is facing then what he (Bob) said or wrote to Charlie is admissible if it furthers the consipracy charge. (Even if Bob never takes the stand, his previous statements and texts, email, etc. stand as evidence against himself and Andy)