I came into this trial late, think that KR was lying her *advertiser censored* off and she drunkenly hit JO.
From what I've seen, I think someone, or someone's, (primarily Higgins but maybe also Albert) beat the crap out of JO in the yard and walked away, either not caring or not knowing how badly he was hurt and that's how he ended up dying in the snow. And Proctor was just slimy or thinblueline enough to take them at their word and not look in the house or interview anyone else present on the day it happened.
It would explain the 'buttdials' and destroyed phones. NOBODY goes out of their way to a mil base just to drop a destroyed sim card and cell in a dumpster (which makes it significantly harder and take longer to look for because it's fed jurisdiction), unless they have something to hide. It would explain Higgins actions that day. And it would explain the unidentified male DNA found on JO clothes. Nobody checked it against Higgins and I wouldn't be surprised if it was a match. Which would blow his 'never interacted with JO' testimony out of the water. He had a crush on KR, he was upset that she was blowing him off after flirting with him and I've seen guys go after someone's BF with a lot less provocation. Certain vocations and subcultures tend to breed a mindset that you can act with impunity because you're special. And some (a tiny minority) of cops get that mindset and it's ignored until someone dies. And it's certainly plausible to me that an ATF agent would take a swing or fight with a 'local cop' over a woman, especially if his pride has been wounded like Higgins was when KR played with him then blew him off.
There was no reason not to ruin the DNA against other men present that night, even the EMTs, if for no other reason than to exclude them, unless someone thought there might be a match that couldn't be innocently explained, or laziness. So, IMO, failing to do so was either sloppy police work or they didn't want to know. Either way, it taints, even more, the investigation.