Oh, I thought I did answer your question. I was just mentioning about the results not being used in court as an aside that Darin could wiggle out of this.
He failed the important Q's due to real guilt, no other reason. IMO
I think Darin caught Darlie right after she stabbed both boys and then they decided together to cover it up. I don't think the murders were his idea. There is too much evidence that Darin and Darlie didn't even know about for them to have set it up for her to take the fall. I don't think they are that smart. They'd have to know that if they cleaned up the blood in the sink and around the sink then Luminol would pick it up. Does that sound like something they would know?
The reason I believe Darin did not help plan the murders is his description of when he first ran downstairs. He describes things as moving in slo-mo, he's confused, he thinks the glass table has fallen and cut Devon, he doesn't even see that Damon is also injured. These are very typical trauma reactions. Voices sound like Charlie Brown's teacher, that sort of thing. Once he got down there, I don't know when he figured things out or what he did once he did figure it out. That part of his statement is the only believable part to me. I don't think it took him long to catch on.