Once DA takes the stand against ZA it opens up pretty much everything he has said to investigators for the defense to use on cross examination.......which means those early statements DA gave to the TBI can be used by either side to help their case.
If DA changes any details from the earlier statements he gave to LE....ZA's attorney is going to use that to try and impeach anything DA is testifying about or gave statements regarding.
If DA implicates himself in any of those statements...or while on the witness stand the defense will pounce on this and try and claim DA was the lead actor in these crimes and create reasonable doubt in the minds of the jurors about their clients involvement in certain aspects of the crimes.
Also if DA implicates himself in any way while on the witness stand this will be used against him at his own trial....If DA is mentally limited this is going to be a slaughter when the defense attorneys get him on cross.
DA can get up on the stand against ZA and change the whole narrative of his statements ....this kind of thing happens all the time.Then the jury has to figure out which (if any) version of the events are what actually happened.But if DA varies too much from his original statements and there is no evidence to back up his current or past account of the events,I'm not sure how a jury could accurately determine which version to go with.
DA is now claiming his early statements were coerced.Meaning it is unlikely his version of events today is matching the version he gave TBI at the onset of the investigation.
What DA probably said in his original statements, and what he probably will say on the stand, is that he was an incidental observer of events that happened at his brothers house. You are not under obligation to intervene if you observe a criminal act, so that in itself will not incriminate him. But, if JA is acquitted, the state will say that he was there as evidenced by his own statements but that his claims that he was not directly involved in the criminality are lies.
In JA's trial the cross-examination can't try to claim that he was also involved in criminality because that would be an admission that their own clients were also involved. Instead they will attempt to say that everything DA said was made up. Later on, when DA goes on trial himself, whatever went on in cross examination in the first trial will not affect the second trial. The only thing that would be addressed would be the original evidence in form of statements made by DA to the TBI placing himself at the scene, the findings of the jury in the first trial will not be entered as evidence in the second trial. The only way for DA to avoid being convicted is if JA is first convicted on the basis of his testimony. If that happens the prosecutor can't then say that DA placed himself there but lied about further involvement in HB's death in order to shift blame, because if they did it would mean that they accepted that DA lied in his statements, and that in turn would lead to JA's conviction being overturned. The prosecutor would have to accept that DA's account is truthful, and that would mean that they would have to drop charges or at the very most convict him of obstruction or something like that, which would probably get a sentence of time served.
As long as DA says the same thing in JA's trial as he said in the original TBI interviews, it doesn't matter because irrespective of what he said on the stand, his original TBI interview still stands as evidence. So he would not be implicating himself anymore than he already has.
If DA gets up on the stand and says something completely different and denies everything, then when his own trial comes up his TBI interview will be entered into evidence and he will be convicted. So if he testifies, he has to at least repeat what he originally told the TBI. Or go to death row.
Right now DA isn't claiming anything, statements and claims will come indirectly through his lawyer or family. He can't be impeached on that basis (because it is hearsay, and anything he told his lawyer is privileged). They could say anything they like, but the only thing that will count in court is what DA said in statements to LE and what he says on the witness stand (if he testifies).
My guess of how all this worked out is that DA originally made a bunch of incriminating statements regarding his brother and friends while the TBI was applying pressure on him, probably in the course of investigating unrelated crimes. This was probably recorded in the form of formal written and/or recorded statements. Then later on when they were talking to him it became clear that he was vacillating and becoming more vague, leading them to believe he potentially could say something completely different on the stand, and that would be a crippling blow to their case if the case was essentially based on his allegations. Charging him with the same crime was a method to get him on the same page as them, and to provide them with a big stick to ensure that he stuck to message. And the sneaky part about it is that even if he did recant it wouldn't matter since if he was charged under the same crimes and stood trial along with the other two, they would not need him on the stand - they could just use his earlier statements as testimony. By splitting the cases they get to use him as a trial witness (which is stronger evidence than the statements alone). So if you look at the progression of events in this case there is a tactical logic in what they are doing.