OK, but let's assume you were actually in the position of having to assemble and detonate said nuclear weapon. How much of your brain would actually be available to deal with the task at hand, and how much is busy thinking "WAAAAAAHHHHH!!!"? (Or whatever...) I'm thinking under those circumstances, mustering an 8th-grade level of concentration might be a bit of a trick.... 
Yes. Mostly that.
Also, consider the kind of person who volunteers for such a duty.
A co-worker of mine use to be with the Marine Nuclear Ordinance Platoon, and specialized in "tactical" nukes. He mentioned a few times how the guys he worked with weren't mentally stable, for a reason. They had to employ people who actually would strap a suitcase bomb to themselves, jump out of a plane, and detonate it. Understandably these guys weren't mental giants either.
Well, don't believe everything you're told by ex-Marines. And realize that the system evolved over time, of course -- you didn't mention during exactly what period your co-worker was talking about. Things were a lot looser in the early 50's than they were even by the mid-60's.
By the time I was in the Air Force in the mid-80's, the Personal Reliability Program could (and did) remove people from the nuclear chain of command for "abnormalities" no more serious than insomnia.
Good points, and very true.
The Marine in question was in for 30 years, the last of which would have been around 2000. So we'll guess that he was playing with nukes around 1980.
As for mental in stability, nothing debilitating or terribly creative. Just the kind of instability that keeps one from having successful personal relationships: mild depression, mild bi-polar disorder, addictive personality, impulse and aggression control, low self-esteem, etc.
