I can answer the last one. The US would finally drop the black-powder single-shot Trapdoor Springfield ( the early model of which it had adopted in 1866) in 1892, replacing it with the bolt action smokeless-powder Krag Jorgensen. That would be standard issue for the Marines until 1895, when the Navy and Marines adopted the Lee Straight-pull. The Lee was in many ways an advanced design, most notably in that the 6mm Lee cartridge was lighter: it traded projectile weight for higher velocity- something that would later be a feature of assault rifles. It was carried by the Marines in Cuba in the Spanish American War, and was used by Marine guards in the Boxer Rebellion. It also had some flaws ( if the bolt release is pushed and the bolt withdrawn, the extractor can just fall off the bolt and out onto the ground...or down, down down into the water) . but in any case 1898 the War Dept decided that it was better if all rifles in service used the same cartridge and for a few years the Marines had Krags, again ( though some apparently had preferred the Krag to the Lee before) . But a general review of the campaign in Cuba resulted in the War Dept finding significant limitation in the Krag also, compared to the Mausers used by the Spanish troops, and a more Mauser-like rifle was designed by 1903, and adopted in 1906, the Springfield 1903.

The general review of the Cuban campaign also exposed weaknesses in the militia system: some militia troops had been badly trained and very poorly equipped, some with Trapdoor rifles. As a result, the militias were transformed, by the Dick Act of 1903, into what would become the National Guard, with Federal training and equipment similar to regular US Army units.

Manuals for these guns are online.

Description and Rules for the Management of the Springfield Rifle and Carbine ( the Trapdoor)

Description and Rules for the Management of the US Rifle and Carbine (the Krag)

The United States Navy Rifle ( the Lee)