Several questions about plot

Well to be honest for a few years before I was able to obtain a home video version to re-watch at my convenience, I wondered the same thing, and in defense of the original post writer here the film sets up the scene in a manner that is somewhat ambiguous about what happens -- After Bill Lusk (actor Henry Beckman, one of my favorite Canadian character actors) shoots the decoy coat, Charles Bronson points his rifle right into the side of the old man's face, and he is clearly wearing a sheathed knife on his hip. We then cut to Lee Marvin and Andrew Stevens watching from the opposite treeline, Marvin says he's going in after Bronson and then we see Bill Lusk, dressed in Johnson's clothing and carrying his gear, come hopping out of the treeline. BUT his face is completely obscured by the hood and angle from which he is filmed, so there's no way to tell whether or not something more had happened to him during his confrontation with Bronson. Marvin yells for him to stop, Lusk fires at Marvin, and POW! Marvin zings him with one peach of a shot.

It's actually only now with the clarity of the Anchor Bay DVD that you can clearly see a "spatter pattern" on the snow next to the body that does indeed show conclusively that whatever happened to the guy's face took place when Lee Marvin shot him. However, it is kind of a stretch to state that a solid slug fired from what appears to be a medium caliber bolt action rifle from a few hundred yards away would do so much damage to a man's face as to render him unrecognizable. As you state, Andrew Stevens does indeed say that Lusk's whole face was blown off his head, and since the camera never shows us the result (a missed opportunity: imagine what a great gore scene that would have made!!) we can only take their word for it.

But it doesn't seem likely that a single bullet would literally tear someone's entire face right off their head. A shotgun blast perhaps, but Marvin is clearly using a bolt action rifle which would have been firing lead slugs.