"What happens if DA plays along and testifies just like the prosecution wants but ZA gets acquitted anyhow? Then DA goes to trial and gets convicted on the basis of his earlier statements he already made to the TBI." DA's atty won't let him testify unless he has a deal. The downside is too great, and no upside. And LE won't want to go to trial without a deal to help guarantee he testifies, with testimony they can reasonably predict. But no one can be (or is) asked to guarantee an outcome - all DAs/attys understand the unpredictable nature of trials and juries.