CCD - Joint Rotation Constraints (2nd Attempt)

While I have struggled to find IK sources which describe this problem or solutions to it, further reading and involvement in robotics/maths forums did indicate some potential solutions.

Using these prompts and research from recent weeks I have made multiple more attempts to rewrite my constraint method and CCD script itself to keep the CCD function, while allowing joints to rotate within their defined boundaries.

Alternate Attempts 

CCD - Joint Rotation Constraints, Further Research

Euler Angles 

Euler angles describe the orientation of rigid body in a within a static coordinate system.
“Any orientation can be decomposed into three elemental rotations”: x (yaw), y (pitch) and z (roll). These three elemental angles can then be concatenated to yield the desired orientation" (Kenwright, 2013, p. 4).
Because each angle is decomposed into elemental axis rotations, using Euler angles within an IK solution would make limiting specific joints degrees-of-freedom to explicit rotations straightforward.
In a 2D IK solution this would provide results as intended, though in a 3D IK solutions gimbal lock would have significant negative impact on the functionality of the algorithm.

Figure 1: no aligned axis, no Gimbal lock (Wikipedia Contributors, 2019)

Figure 2: aligned axis, Gimbal lock (Wikipedia Contributors, 2019)