Just went back and looked, nothing mentioned in your post #2, #6, or #14.
I’m sorry about your experience. I would have done things differently, but that’s not me saying you did anything wrong.
It would appear, you shot him quartering to you, to what angle only a video would show. The arrow probably hit that rib and deflected it to more of an angle than it already was, causing a severe gut shot. Bummer deal.
My only thought for next time, work out before next summer and bump your poundage up to gain some speed, use a heavier arrow setup, and punch that front shoulder. Or just wait for perfectly broadside/quartering away shots.
But that’s just my opinion.