Metals do not like water nor humidity.
The brass will darken over time, this is normal, you can clean it or accept its evolution enjoing Wabi Sabi culture.
The joints will become smoother with use. Exercise your Kinetic Armature to keep in shape.
It is possible for the screws to go loose or not to offer the desired pressure overtime. Adjust them to your needs but avoid forcing them beyond material limits. Consider using threadlock if you need it.
Brass reacts with foam latex. If you are planning to make your puppet with that material, remember to recover all the armature with teflon tape or silicone before cooking it or your latex will get damaged.