I am quite confident that the prosecution has an understanding of what Katie is going to say in this Proffer. The outcome of this Proffer transaction, if successful, will result in both sides receiving something of value and, as such, neither side is going to want to proceed without some assurance that Proffer will be sufficient to achieve the desired result. Typically, defense counsel will approach the prosecution first by saying that the defendant might be willing to cooperate in exchange for either a reduced sentence or some sort of accommodation in her incarceration (perhaps a different prison or one with better food and amenities).
Experienced lawyers are well versed in this dance whereby they never promise anything definite while, at the same time, making very clear what they are willing to do. For us to get to this stage, Cappleman almost certainly made it clear to Kawass and crew what she would be willing to do if Katie could come through with credible and usable evidence/information. This would result in Katie's attorneys providing a hypothetical example of the information she "might" be willing to provide in order to assure themselves that it would satisfy the State. For instance, Kawass might have said, "What if Katie has a family member who will corroborate Wendi's involvement?" The defense is at a disadvantage now with Katie already sentenced, so it isn't in their interest to hold back or play coy. They will lay out the most shiny object possible (always as a hypothetical) in order to see if the prosecution is interested and willing to deal.
Concomitantly, the prosecutor will also always speak in hypotheticals, such as "I might be willing to reduce life without parole to 50 years." For the judge to enter an order setting up a formal Proffer means that both sides haven't formally committed to anything, but they understand precisely what the other is offering and the formal Proffer will be taken under oath. So, there almost certainly isn't any big mystery on the part of the prosecution as to what Katie is going to say. They know generally what she is claiming and the Proffer is the final step whereby the defendant is deposed and questioned under oath as to the material issues. Rest assured there have been MANY discussions between counsel over the past months in order to arrive at this juncture.
A very interesting element to this is the almost universal requirement that the defendant agrees to testify truthfully in response to ANY questions asked. If the defendant lies or refuses to answer a question, the deal can be rescinded by the prosecution. This is similar to what is occurring with Alan Weisselburg who is testifying against the Trump Organization right now. He pled guilty and the State agreed to reduce his sentence substantially -- but only if he testified truthfully and answered any question truthfully at trial. He doesn't want to be testifying, but he is having to give uncomfortable answers because failing to do so would see his cushy sentence jacked up to 20 years or so.
Katie and her counsel know what the State is willing to do and she obviously wants it badly. To get it will require her providing the information her counsel "suggested" she would and also she will have to answer any questions the prosecution has. I expect Georgia to inquire widely on issues that include who paid for the defense (unless Kawass struck a deal ahead of time that the State will not inquire into that matter).
One final note regarding the speculation that Katie might be going pro se now. I would bet my house this is not the case. I cannot imagine the judge would permit a defendant to make a proffer without counsel and I cannot imagine Georgia would be willing to negotiate a proffer with an unrepresented defendant. I'm quite confident Katie has attorneys and my money would be on Kawass, if only so she can monitor and attempt to control the narrative. After all, she knows that if this goes through the press will be hounding her as to why she didn't encourage Katie to strike a deal years ago. There are obviously huge questions about how Katie was represented and I suspect Kawass and possibly DeCoste are going to represent Katie now to protect themselves to whatever extent they can.
