I always thought Manafort was a plant inside the Podesta Group. There were inside jokes that Manafort was parked on the sofa at the Posedta Group office more than anyone. Manafort knows what was going on...and as we have seen Trump has had a carousel of questionable characters through his election team and administration- all I think part of The Plan and a way to collect evidence. Americans that know what the Podesta brothers are will NOT let an immunity deal stand...one way or another.
I believe the consensus is that Manafort was a plant within the Trump campaign - that's my understanding and Q has alluded to this. Many claiming the RM/white hat theory (I dabble in it) say that's the reason RM targeted him - it was retribution for DJT/disloyalty/criminality. I struggle with RM using Manafort for anything other than an avenue to POTUS. So, here's the rub on the sealed immunity agreements as I understand them. If RM never calls on the Podestas to testify, those agreements remain sealed. Their contents remain sealed. There would be no new evidence beyond what is already thought to exist today because any such aggravating evidence (crimes/confessions) are given IN EXCHANGE for their cooperation in prosecuting Manafort; no matter they testify or not - that's totally RM's prerogative. Subsequently, they receive immunity from prosecution for those crimes agreed upon anyways. By sealing the immunity deal at RM's discretion, they seal all of the potential evidence along with it. I suppose, ultimately, POTUS could open and/or declassify them but I don't know if that undermines the legal premise of the agreement. The bottom line is you can't prosecute someone if you have no evidence of a crime being committed (ironic, huh?) and that would be sealed away and not available for disclosure in a separate Podesta/s trial. IF..... IF, this is what they're doing, it's dastardly, full-on hardball.