Rotating On Two Axes Independantly

Ok, so this could be hard for me to explain, but I will try my best and make use of extensive images. This is a 3-Dimensional math problem, and I am having trouble finding the answer.

So basically, I have written a script that creates a "ring" of orbiting GameObjects (in my example, several Spheres) that are instantiated as a Child under a central GameObject (in this example, a Cube).

The cube's origin is 0,0,0 with no rotation. The spheres are generated equidistant from the origin Cube when given a distance (public int dist) at an angle calculated based on the number of objects being generated (public int num). This script is placed on the Cube object:

 Vector3 origin = transform.position;
         for (int i = 0; i < num; i++){
             int angle = 360 / num;
             int a = i * angle;
             var pos = NewCircle(origin, dist, a);
             GameObject newPrefab = Instantiate(prefab, pos, Quaternion.identity);
             newPrefab.transform.parent = transform;
         }

and

 Vector3 NewCircle(Vector3 center, float radius, int a)
     {
         Debug.Log(a);
         float ang = a;
         Vector3 pos;
         pos.x = center.x + radius * Mathf.Sin(ang * Mathf.Deg2Rad);
         pos.y = center.y + radius * Mathf.Cos(ang * Mathf.Deg2Rad);
         pos.z = center.z;
         return pos;
     }

So that's the basic creation code. It is built on the X and Y coordinates of the scene and it ends up looking like this:

The problem comes when I start rotating these objects.

The goal is for this object to rotate on the X and Z axes, with the Y rotation remaining unchanged (at 0) so when Trail Renderers are added to the Sphere objects, it creates a very particular pattern.

The problem is, I can't rotate on the X and Z axes without affecting the angle on the Y axis. I came up with a solution that technically achieves this. However, the Trail Renderers do not "trail" on the Y axis. Why? Because, instead, I have positioned the camera and rotated IT on the Y axis, which gives the illusion of the object rotating. However, while it shows the correct positioning and rotation of the object, the effect is not what I want. Rotation code:

 void Update () {
         targetRotation *= Quaternion.Euler(0, 0, 2);
         transform.rotation= Quaternion.Slerp (transform.rotation, targetRotation , 10 * Time.deltaTime);
     }

Basically, the "ring" rotates counter clockwise, on the Z axis (See previous screenshot), while the Camera, positioned under the ring, looking up (Orthographic; Rotation: Y=90;), rotates on its local Z axis, or rather, the Scene's X axis.

In-Game, the Object appears to be rotating along the Scene's X axis, but really is just the camera rotating:

So, as you can see above, the Trail Renderers do not render along the X rotational plane, because the object, is not rotating along it. However, If I add X rotation to the Objects rotational code:

 void Update () {
         targetRotation *= Quaternion.Euler(2, 0, 2);
         transform.rotation= Quaternion.Slerp (transform.rotation, targetRotation , 10 * Time.deltaTime);
     }

the rotational planes combine, and begin affecting the Y axis' angle of rotation. It needs to stay at 0.

I am sure someone smarter than I has a mathematical way of interpolating the Y axis to remain at 0. Any help is much appreciated.

Thank you in advance.