Working on those guns requires knowledge of how to use slave pins (ooops, now called "helper" pins to be politically correct). Otherwise, you can go crazy in a hurry.
As "collector" and I have discussed before, I see nothing wrong with using regular factory smokeless loads for the few shots anyone will ever fire from those old guns, especially as BP loads will have to be made up. In any case, those guns were made from cast iron and while of moderate quality when new, they should not be fired to any extent today. (The guns won't "blow up", but more parts will probably break!)
As to the cylinder turning freely, the cylinder stop is part of the trigger, and only engages when the trigger is pulled. When the trigger is forward and the hammer down, the cylinder will rotate freely in the clockwise direction, and sometimes in both directions. In use, that was prevented by the firing pin (hammer nose) resting in the primer of the fired round until the trigger was released and pulled again. The guns were usually carried on the half cock notch (not recommended!), but some safety-conscious owners let the hammer down on a fired case or let the firing pin down between cartridge rims.
The grips, if original, are of a material then called "gutta percha", now generally called "hard rubber." By either name, it was flexible enough when new, but turns brittle with age and cracks easily. Plastic replacements are available for some of the old guns, but not all.
Jim