First of all, from one mother to another who completely understands and feels your pain… at the same time searching for answers to questions, I am also learning through the process regarding my daughter’s death case…
As you have, through the same process, while we are grieving not only the loss of our children, and everyday moments, but also bearing the additional process of making the calls, finding additional officials, and compiling evidence for any kind of justice, in want only seems to be an unjust world, as opposed to law enforcement, detectives, and medical examiners who are supposedly trained but fail to do their diligence… only adds to our compounding grief… I am sorry you too, are dealing with similar incompetence…
With that said, the only other advice it seems I can offer you is even with the peaks and valleys of your days, and as I have been told is to keep compiling the evidence, store it properly, and keep fighting!!
If you have not checked and you have access to his phone, laptop, or computer, and you know his information, maybe download any Google account, images, Maps, and activities (Not sure how far these items go back), it may shed some lite into a location and timeline, whereabouts or individual(s) houses or who may have been involved by location compared to known location(s). Same with images, messages, and time stamps.