Why would she need to work out a plea deal when there is no evidence of anything? Remember, that in spite of the shifty nature of everyone involved in the case, there is no evidence at all that anything nefarious happened at the house, or that either SA or BD did anything. HD is just gone. Based on the multiple sightings of her that day and SA's cell call ping timing, the most likely truth is that she left just as he said she did. The window between when he arrived back in CC and when the brother arrived home is just too small for him to have done anything and completely removed all traces of it. If the DA was dumb enough to go to trial with what we know today, that is what the defence attorney would present and the DA would not be able to refute it.
It has allready been established that there wasn't any illegal




at their house. The only potentially illegal stuff was found on someone else's computer in another town. It would be a massive stretch to tie either SA or BD to any of that, basically it is wishfull thinking and that is no way to conduct an investigation.
There simply isn't any case so what could they possibly be charged with?