1. It varied from nation to nation. Some never scalped. Some scalped as a celebration of victory. (Overcoming an opponent in personal combat–a requirement for performing scalping–was rated higher by some war societies than actually killing the enemy and there are numerous stories of persons scalped when overcome, but then recovering and living out the rest of their lives with scars on their heads.) Some scalped in an early version of what later became the Vietnam “body count.” Various European settlers did pay bounties to allied nations for bringing in scalps to prove that they had killed the mutual enemies. (Whether this practice was simply the Europeans taking advantage of a practice that already existed or whether some European with a knowledge of ancient history introduced the practice has been debated.)
2. Among the nations where they celebrated personal victory, they would have been displayed as trophies (as did a number of European-descended settlers who took up the practice). Among nations where they were simply “body count,” they were turned over to whoever was paying out the cash and I do not know what happened to those.
3. It is possible that some nations refrained from night attacks. Whether they did so out of a fear that their afterlife would suffer or whether they did this for the practical reason that night fights among large groups using projectiles are liable to be inconclusive affairs with a certain number of “friendly fire” casualties, I don’t know. Certainly, assigning superstitious motives to the enemy makes a convenient plot point in a movie. (There were, in fact, many attacks by Indians that were launched in the dark, so if any nation had a fear of night fighting, it was not a universally held belief.)
5. The U.S. Army had a traditional faith (at the time) that a soldier who was taking the time to sight on an opponent was more effective than a soldier who was wildly firing as much ammunition as possible. Both the Infantry rifles and cavalry carbines tended (there were exceptions) to be single-shot weapons (although the Army did go to the faster-reloaded breech loading models soon after they became available). At the time, single-shot breech-loading weapons were also less subject to jamming in combat conditions.