Great post. Sleep is one of my favourite things! However I do question one aspect of the last piece of research you mention, which looked at sleep duration and its effect on high-level cognitive abilities. It certainly seems likely that not getting enough sleep would impair various mental functions, but the research doesn't take into account the possibility that it might not be the excessive sleep that impairs reasoning and verbal skills, but that an outside factor might cause both the excessive sleep and the impairment to reasoning and verbal skills.
I'm reading this the morning after I've slept for about 10 and a half hours, and my brain feels like mush. But I don't think the long sleep caused my mushy brain syndrome. I suspect that the effect of a crazy stressful week, followed by four hours of indoor bouldering last night where (egged on by my bouldering friends) I pushed my body to the limit, getting a bloody blister on my hand and a bruised knee in the process, caused the brain mush and triggered the excessive sleep. After that, I had a bath and collapsed into bed, falling asleep as soon as my head hit the pillow. I think the 10.5 hours of sleep was necessary and restorative, and my brain would have been even more mushy without it!