"how the plaintext was obtained" which means it sounds next to impossible FBI can keep their exploit secret indefinitely if anything on an encrypted device is going to be used as evidence in a criminal case.

Maybe, if the defense counsel specifically tries to get that level of detail out on cross. But in the ordinary course of events it probably wouldn't. The jury isn't going to understand or care about the details -- how is eliciting them on cross going to help the defense? Remember the defense lawyer is responsible to his client, he isn't a roving do-gooder.