Because the 911 call was taped, we know Darin discovered the murders during that call, as Darlie explains to him what happened. I don't think he had the time or the wherewithall at that point to decide to plant evidence to help her. He first tried to recessitate Devon.
The sock is an enigma for sure. But who knows what Darlie was thinking. If she had the bizarre mindset that she could stab her children and then blame it on a mystery intruder, then a bizarre action with a sock falls into the same category. It doesn't make sense how or why she planted it, but none of the rest of what she did makes sense either. Like, why wipe Damon's bloody handprint off the sofa? There was blood all over. Why plant a sock with so little blood on it? To prove the intruder went through the laundry room and picked up a sock on his way towards the cut screen? To throw blame on the guy who lived in the house of the yard where the sock was found? It's just a wacko mind doing more wacko things.
The sock is an enigma for sure. But who knows what Darlie was thinking. If she had the bizarre mindset that she could stab her children and then blame it on a mystery intruder, then a bizarre action with a sock falls into the same category. It doesn't make sense how or why she planted it, but none of the rest of what she did makes sense either. Like, why wipe Damon's bloody handprint off the sofa? There was blood all over. Why plant a sock with so little blood on it? To prove the intruder went through the laundry room and picked up a sock on his way towards the cut screen? To throw blame on the guy who lived in the house of the yard where the sock was found? It's just a wacko mind doing more wacko things.