For face posing, I had trouble trying to understand how to get it working for a while in SMH, but then I found out something. Face Poser always overrides expressions if you save a frame with the tool on equip. What you need to do is get the face pose you want, switch to another weapon (I preferably switch to the crowbar, press 1 and then left click) and then record the frame. When you do this, it'll save the face pose so when you go to another frame, edit the face pose there, do the same process (crowbar and record frame) and then use the smh playback command, you'll see a transition from the first face pose to the second. Hope that helps, it took me ages to figure that out. Turns out it's in the FAQ on the addon's main page (for reference:
Q: SMH isn't playing facial animations?
A: If you have the faceposer tool open, it will override any facial changes made when you move between frames with SMH, so you need to unequip the faceposer tool to see the changes.)