How to weight paint character whose mesh is in several [joined] pieces?

Question

Hello all you beautiful Blender-people! I am trying to finalize a character but am having trouble pinning down the bone weights. My character mesh is technically "one mesh" but that mesh has multiple disconnected components. That is, the eyes, eyebrows, nose, mustache, collar, hands and feet are not vertex-merged with the rest of the body. That can be changed for at least some pieces if it must be, but let me describe the problem:

I am getting severe deformations in the collar when the neck rotates (it shrinks to a singularity); I can get the eyes and the nose to stick to the head when the head bone is manipulated, but not the neck; Similar story for the mustache.

No matter how paint these components, I run into similar issues.

Version of weight painting that works for all facial features except mustache when NECK is moved, but not the head:

Another version of weight painting that works for all facial features except mustache when HEAD is moved, but not the neck:

What precisely do I need to consider here to make the facial features move as expected? What will keep these components (these are all part of same object) from scaling?

I also have a Youtube video using this rig where I teach people how to mess up their characters (BE NICE PLS b.c. I'm gonna redo it anyway after I figure this out, I'm sure being disconnected pieces is a big part of the problem):

Link to video

EDIT: So I just noticed that the mustache is painted to a knee's pole target, so let's see what happens if I paint that out...

Answer 1

OK! So, cautionary tale for other animators: If you redo your IK bones 50 times, make sure that each time you uncheck "deform", otherwise good luck figuring out what portions of your mesh you are going to get unwanted vertex weights for. In this example, the IK bones at the heels painted the heels (unwanted), but the pole targets offset from the knees painted only the mustache (for some reason, also unwanted). Beware!