Regarding the question of self defense at short range, I can only speculate, as I am not an authority. However, it would seem to me that if you are in a really desperate encounter, you would want to stop your attacker in his tracks. A single large-diameter ball might just do that. I think one or more buckshot might create a mortal wound, but the attacker might have some fight left in him before he expires.
I think wadding or patching would also make a difference. Testing by the Bevel Brothers, published last year, showed something like a 40% loss in velocity for shot loads wadded with loose fiber (e.g. tow) as opposed to card or dense fiber wads fitted to the bore. I expect a half dozen or so buckshot wadded with tow would have a lot of gas blow-by that could be eliminated by using a single ball close to bore size. Couple that with the down range energy loss of the smaller pellets, and the buckshot load in a handgun starts looking really puny.
At really close range, like handshaking distance, I don’t think buck or ball would make much difference… The buck wouldn’t have room to spread.
In shooting smoothbores with shot, whether loaded from the muzzle or the breech, I think it’s a good idea to pattern your gun. What you find may surprise you. I learned this in cowboy action shooting. A lot of guys failed to knock down shotgun “poppers” placed at 8 to 15 yards, and swore the targets were too far, that the shot patterns from their short-barreled, cylinder bored scatterguns opened up too much. I was intrigued, and patterned my own double-barreled coach gun (20” cylinder bored barrels), and found the patterns were a lot tighter than expected. At 8 yards, it was essentially one hole the size of my fist. Adding to the problem, I believe the barrels on the cheap doubles a lot of the guys used were not well regulated, meaning point of impact did not necessarily coincide with point of aim. If the poppers were not going down, it was because the shooter missed.
Just some random thoughts, mostly speculation on my part.
Notchy Bob