Our skull has only one bone, which is able to move, the jawbone. But the facial muscles will allow us 1000's of different poses. And the stretching of the skin is not really local ... go in front of a mirror and look what happens to the ears, when you move your jaw.
So each way could only be an approximation of the real world. Shape-keys (using a start and end point and changing linear between these points) and also bones (using a weighting of vertices near the bones and also working linear) are a way to make it look real. Another approach is this:
https://www.youtube.com/watch?v=tuZMMZ8vbNkBut I'm also far away from this perfection and for a character posing somewhere in the background this does not matter. But of course, I'm also dreaming, that in some times we will all be able to do that ...
Your teeth are better than the original version. I always lower the teeth after importing them in Blender and put them a tiny bit forward. If you want, you can select each tooth and assign another material (like pearl maybe). Or simply add more gloss in Blender for the complete teeth, the internal part of the mouth normally is wet, so it is okay to work like this.
With the MOCAP skeleton my experience is not so good, I need more details normally ... especially the "arm behind head" fashion pose does not really work without weird deformation
Until now my way is to put all bones in a face-group and then I made a facial expression library in Blender (pose-library). To place a character, I first assign a pose from my pose-libs to the whole skeleton (e.g. sitting-poses), then I select all face bones and assign a pose from the facial expressions pose-library.
If you make facial pose libraries in Blender be aware that what is a smile for one character could be sth. completely different for another character ...
... so you need a library per character for the face.