Stephen Stills has a hearing loss issue that began when he was a young child and has worsened over the years. His dysarthria is related to that. It has nothing to do with his cognitive abilities.
No, I don't mean the sound/tone of his voice, I mean he sometimes seems to be having a different conversation. Maybe it's related and I'm being unfair. Crosby, by way of example, is SO erudite and well-spoken, it's possible it's just perception.
I watched that interview with Dan Rather - though it was years ago - I cannot recall if he was wearing his hearing aids or not. Sometimes people with pretty profound hearing loss can lose bits and pieces of conversations especially when there are more than one person talking. I suppose that could make it easy for it to seem like he was having a "different conversation." His hearing loss issues are not a secret certainly, but I'm not aware that he talks very openly about it. As he's had it to a degree since childhood it could be sort of normalized for him and for his close friends and associates.
And yes, SS did a plethora of drugs back in the day. Couple that with normal age-related cognitive declines, it wouldn't be unreasonable to assume he has some damage there. Having a close family friend with a daughter who has hearing loss it gets upsetting when people make assumptions that she isn't intelligent because she has a speech impediment based on that hearing loss. I just wouldn't make assumptions that SS is suffering from brain damage any more than any of the other guys in the group.
I also agree that David Crosby is pretty masterful at the art of flowery speeches and is a very smart guy for someone who should've been dead about a dozen times in his life.
The issue between the band mates shunning of DC is honestly because of DC's lack of a social filter. He'd be the first to admit he's an ass and though he has apologized numerous times, it seems the bridges have been burned with little hopes to rebuild them. It started with Neil Young's choice of girlfriend and now wife, Darryl Hannah, who famously had a row with Crosby's long time friend, Jackson Browne back in the 90s IIRC. Then Graham Nash dumped his wife for a younger model - Crosby and Nash married their brides in the same ceremony back in the late 80s. Crosby is still married to his wife, Jan Dancer. No doubt DC had some feelings around Nash dumping his wife after nearly 40 years and said some hurtful things to the press about it. Nash has yet to forgive him and very likely never will. It's sad to a lot of people that CSN and CSNY will never be again. But I'm not one of them. I like DC's musical output since they broke up and I think he is doing just fine without them. I'd rather remember them in their heyday.
Speaking of memories, I just watched the 50th anniversary of Woodstock documentary from NPR. It's excellent. I wanted more music but I learned a lot of things I never knew before about those few days on Yasgur's farm. Highly recommended.