I guess my logic has these parts
- due to the anatomy of the fist, spreading over all 4 options isn't an option as they don't form a line. Only the 2 big ones, OR the 3 smaller ones do approximately form a line; thereof the two choices.
- from own experience of body conditioning I feel that the more painful punches are where you feel the knuckles digging into you. So for this reason, when I throw a punch, I don't think of throwing a fist, but I aim with the knuckles. So in my mind I am firing my knuckles most of the time. But this is NOT how I think if I punch a boxing ball at the amusementpark, then I found indeed spread the power as muych as possible and just add as much body power as possible behind.
- I frequently also step aside, and hooke behind the guarding arm - back ribs, kidneys, or try to hit the nerve joint in the shoulders back side with one knuckle... and there a vertical punch would certainly hit with fingers unless I am VERY close, I often the hit with one or two knuckles as a angled hook. But the whole body is not behind this punch.
- as I want to delivery pain, if you distribute the force on more knuckles, there is less pressure on each knuckle. So delivering the same knuckle pain from 3 knuckles, requires triple the force? You do not need as much power with one knuckle to get effect. A knuckle strike served to give pain, not to make the head spin. Similar precision as you do with knuckles can be done with the heels when kicking. It stuns the opponent for a bit, not knocking them out.
I get the risk of boxers fracture with a pure horizontal fist if you hit the hard bones in the head or jaw.
That said I understand there are different ways that may all work. I just seek to understand the "logic" of each perspective. Perhaps the issue is also that my own limitd experice is not drawn from punch the head, but the body. I will experiment a bit more with vertical on the bag to see how it feels.