Yes you can modify the bases and save them as custom but its just a regular armature, you can create your own from scratch and set the display mode to envelope drawing.
Really? this might be a bug, do you see any error message on your console?
It really takes long for Boolean operations on dense meshes, it’s a limitation of blender itself. But there is the mask split tool which performs just as well for dense meshes but works by separating masked from unmasked areas.
Forgot to tell, The shortcut for the pie menu is ALT + W