1. With the possible exception of the Sharps, no breechloaders of the period had the range capability of muzzle loaders -- so tactics would have easily compensated for any theoritical advantage in weaponry.
Agree with the range. Tactics assume a set piece battle, warfare is not that predictable. There are battles won by natives with pointy spears over firearms, but the usual outcome, given good Generalship, that an arms advantage is an advantage that contributes decisively to winning.
2. The supposed "advantages" of breechloaders indicate a rather naive idea of combat -- of that period, or any other period.
I have read enough personnel accounts of civil war soldiers, one in particular was a Trooper who served under Custer, was very proud of it, and he carried a Spencer. The Spencer was a real advantage for the troops armed with it. So often battle was an individual against another individual, even in a crowd. The guy who had the most shots had the advantage.
Compare single shot pistols to the Colt revolver. The single shot pistol disappeared quite quickly from battlefields.
4. Most soldiers weren't capable of using muzzle loaders very effectively, and their officers paid scant attention to weapons training.
True. True in every war we have ever been engaged, and true today. Private Jessica went to war, and did not know how to clear her weapon. She said "it jammed". Cost cutters get ahead in peacetime. One of the things they do, to free up money, is cut training.
As to the best military rifle of 1875, I have a Martini Henry, a Mauser M71/84. Handled rolling blocks. All are good designs. You really have to handle a Martini Henry to understand what an excellent design it is. Very simple, very safe, impossible to put together wrong, easy to clean. But the Rolling Block was the most exported action, so I guess that makes it the "best".