It sounds like you've done all you can to find him. If you searched and then finally quit the general direction and then grid searched as best you could for the days you did, the bull is not dead. I've been bowhunting for 25 years as of this coming season. I've got over 100 animals if not more under my archery belt. There are just too many variables in the hit. Sure the lungs should have collapsed as stated. But if the hide was pulled back then as the animal retreats the hole in the hide and in the cavity are not in line and sealed off again. So many broadheads found years later prove how tough animals are. Even killed a gut shot!! deer a year later. Elk are tougher than deer are. Then there is the issue of the hollow spot. Yep its there and depends on how inflated the lungs are. Also just because you hit a lung or two, you may not hit it vitally enough. We've shot animals just as described that have been seen the day after or the evening after the morning shot that you cannot even tell much of where the hit was. The positive side to archery is that if you do your true best, stick too it, look for a couple days, have help etc.... I think odds are probably about 80-20 that he did not die. The broadhead is clean, a sharp clean surgical cut, and heals extremely quickly. VS the rough damage that bullets create. Those wounds are much more likely to become septic and create a seriously deadly wound later on. The only things that create doubt about any "non instantly fatal" wound to me are cutting anything(arrow or bullet) that deals with the esophagus, or gut cavities. Those harbor lots of bacteria and can create the infection that will slowly kill later on. But rest assured that most archery hunters look so hard that if they kill, they find. (not counting slobs). And that there is still a large part of gun hunters that think if the animal doesn't fall at the shot they drive on(note I said drive because these are also slobs). I'm not saying anything bad about either. Just there are many more gun hunters, and archery takes a lot of dedication. IE if you take up archery you usually learn a lot about everything. Gun hunting often starts by borrowing a gun, and going with a buddy. You don't know any better, and learn slowly. I only failed to loose deer with a rifle when I started because I had a super mentor. Now lets go back to the hit. When did the bull break the arrow off? What track did the arrow take? IE you never know what path the arrow takes until you recover the animal. I've seen one deer shot from a tree that was hit behind the shoulder but before the arrow could penetrate the ribs, the front leg moved, and kicked the arrow forward and UP. Ended up accidentally severing the neck. Then we have a doe on video. Entry from behind and above is perfect. Perfect to take out the heart. Shot, looked and arrow. Covered with guts????. Video looked perfect. Waited normal 8 hours due to sign. Results were dead deer within 150 yards or less of hit. But almost pure gut shot. Nicked one lung (not enough to cause death) but somehow it ended up going backwards into the guts. Watched video in super slow motion. You can then tell the arrow takes a wild path as the deer recoils from the shot.
My end thoughts(worth 2 cents or less or whatever you value them as- remember over 100 archery kills and was a guide in an area where our clients often killed 300 deer a year between 2 to 3 guides each year....)are that could the elk be dead? Yep anything is possible. But due to your efforts I seriously think that someone else will take him this year and find the broadhead OR you'll have a chance again next year. PS got video stashed somewhere of a couple of pigs, one with an arrow through its head and another with a high hit a few days after the hits eating corn on the sendero again, non the worse for the wear.
I know you'll loose sleep over it, but don't let it get you too down. Feel you need to be super ethical, quit hunting this year and don't shoot another. Start over again next year.
Rest easy buddy, it happens too us all. Rifle, bow, pistol, muzzleloader.......
Jeff