Someone encouraged me to go back and see if I could use scavengers to lead me to the remains of the buck. It was a long drive back to that area, and a mile hike back into the swamps, but I found him! The wolves/coyotes/eagles/ravens had eaten most of him.
Just as the sky was starting to get light I heard an eagle call, perched in a tree nearby. As shooting light neared I could hear ravens. Once I could see well, I carefully walked towards the ravens until I could barely spot them and see that some looked to be landing. As I walked through the thick cover I kept a distant high treetop near the ravens in sight, and when I got close ravens started flying up and I knew my buck would be there. Then I spotted deer hair and took a few more steps to see what was left of him, piled up in the swamp.
It was apparent he had died running, and had been dead most of the time the three of us had looked for him the first day. He had gone only about 50 yards from the last blood. We'd searched well beyond and around him for hours but just hadn't stumbled onto him. As they often do at the end he had veered off at a random angle and ended up in a thick black spruce and alder bog. If he had kept bleeding obviously he would have been easy to find. Or if there had been snow. Or ground conditions where tracks or scuffs had been been visible. Or if dogs had been legal.
We were right in deducing that he was hit forward and low. But I thought he was hit even lower than he actually was. He was hit through the left front shoulder, but the bullet had angled so it didn't hit the opposite shoulder and had barely missed the lungs. Due to reported impending rain I'd tracked him right away or he would likely have been dead in his first bed.
The big mistake was shooting at a running buck that far away on dry ground. But I am very thankful to have found him and to know he didn't suffer. As you can see he was a massive 11-point swamp buck, the base of his antlers are nearly black.