I used to build my clubs when I was younger so I know a little on the subject. Hosels and shafts are connected with a strong epoxy. Iron shafts have generally a .370 parallel tip or .355 taper tip. I mainly worked with .370 tips; however, many club manufacturers use the .355 or taper shafts. My guess would be because it is more secure.
As far as the issue you are concerned about, don't be, it happens quite often and most times it simply needs slid back down (more like forced on occasion). The Ferrule (black part) is there to simply clean up the look of the club.
I have slid the ferrules back with everything from pliers, a 30lb vice, to just bear handing it down. There is more then one way to skin a cat. Please do not hurt yourself trying any of these homemade methods.
Also, I would not be slightest bit scared that the head will fly off.
Hope this helps,