….asked where the TMJ joint would fit into Coach Boyle’s Joint by Joint Theory….

I don't know what Neuromuscular Dentistry is, but it probably makes a whole lot of sense.
I will have to look at the algorithm, but my impression is that the jaw is a stable joint.
Here’s why, and it’s just semantics because the fact of the matter is that all joints require both mobility and stability.
Using Sahrmann as a foundation, pain abounds from something that moves too much. So when we consider “stable” joints, these are the joint systems (not always just 1 joint) that have problems when there is hypermobility. Stability can be defined as control of a segment in the presence of potential change. That does not mean lockdown isometric mid-range “stability.” Based off the hypermobility prevalence of TMJ, I would consider it a stable joint system.
And if we use the O-A joint as mobile, the TMJ is supposed to be stable.
The cervical spine is really debatable, and it simply lends to the credence that the Joint by Joint is more a theory and constructive way to think more than a steadfast biomechanical map.
But the fact of the matter is that it does work out in the alternating fashion almost any way you crunch it.