This is a better reply Simon. Now I can read, in context, what you’re doing in relation to the steps I’ve proposed.
[Simon Faris] “Yet after reading, it seems that is a null point due to the hierarchy of the stacked layers. FG being atop.”
FG must always be at the top of the Layer Controls Panel. To be safe, use the tracked FG layer and not the tighter spline of the FG which I assume you linked to the originally tracked FG.
Selecting Subtract Matte is not required as mocha is programmed to use any layer above the currently tracked layer as a Subtract Matte.
[Simon Faris] “Using Subtract Matte on the foreground even though it is atop the stack? Imagineer videos say NO.”
You must have misunderstood this. What Imagineer meant was what I said in the previous para on the redundancy of explicitly specifying a layer as a Subtract Matte.
Here are a couple more tips –
1) You can view mocha’s Track Matte to see exactly which area(s) mocha tracks. mocha’s nomenclature for a Track Matte is quite different from AE’s or other apps. For mocha, a Track Matte defines the tracking area.
See the top-left of the screen cap below on how to set up the Mattes menu to view the Track Matte of selected layers.

The canvas area clearly shows that the head of the FG has been excluded from the Track Matte. This means that the FG (head) area will not be tracked.
The last tip is to set/resize your insert clip to fit the size of the tracking comp. Then precomp the insert layer PRIOR to applying mocha’s tracking data.
BTW, use the Adjust Track only as and when required, ie when your track is less perfect.
HTH
RoRK
Intensive mocha & AE Training in Singapore and Other Dangerous Locations
Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects CS6 ACE