Why did Moro only manage to take off Eboshi's one arm? She can never fire a rifle again.
Well, it wasn't Moro at that point. It was her zombified head taking a last crack at Eboshi before dying. There wasn't much thought involved at that stage.
If she can design a rifle women can fire, it's not much of a stretch to think she can design a pistol.
A pistol is a lot more difficult to construct than a rifle, and if memory serves it was Eboshi's dominant arm that got chomped off.
she might be able to design a pistol, but Moro didn't know that.
Even if she could fire a pistol one handed, could she load one? These are muzzle loaded matchlock weapons. It can be hard for people with two arms to load them well. She would always have to have someone there to load it for her. Her loss of aim due to her disablity, the wasted person on just reloading for her, and her sense of independence and efficiency would lead her to just stay back in town and administrate.
Unlikely, she already has people loading for her, judging by the fight scenes. Considering her aim, it is probably a good choice.
A subtle one. When Ashitaka sees the Samurai massacre near the beginning, you see a warrior cut a mans arm off. Almost right after that another soldier shouts "They're unarmed, move in!"