The controller was one of the good ones really trying the help the pilot as well.
If you ever hear a controller tell you the altimeter setting out of the blue, you probably just blew an altitude restriction and they don't want to have an audio record that can result in action. "You've exceeded the class C" or "check altitude" is admitting a problem, "Altimeter 2983" is not admitting anything, and allows you to correct a problem without making a bigger deal about it or requiring a post flight phone call.
A altimeter setting of a nearby airport when in the flight levels is really not necessary, as your altimeter is supposed to be set to standard 29.92 above 18,000 feet.
I wouldn't think it was a rapid decompression crash, but possibly a never compressed crash, the cabin pressure malfunctioning or never switched on with the slow onset of hypoxia during the climb, ending with the pilot slumping over the controls. Another possibility is a medical issue (stroke, heart attack, etc.) with the pilot. I agree with Burt Reynolds that it sounds like incapacitation.