No. The levels are new objects in their own collection; your mesh, armature, actions, pose and frame are left as they were. Building again replaces only what SkinLOD made.

Because that is where LODs break. A decimated character is often a fraction of a millimetre off in its rest pose and several millimetres off mid-stride. Every level is shaped and judged on your actions.

Unity keeps at most four bones per vertex (fewer if you ask). The usual way keeps the strongest ones and drops the rest. SkinLOD chooses the bones and weights that make each vertex follow the full mesh most closely over your frames, and keeps the plain cut where that is closer.

From the measured error: a level is shown once the character is small enough on screen that its error in motion covers no more than the pixel budget you set (1 pixel at 1080p by default).

The LOD Group and skin quality do not depend on the rig type. The Unity measurement plays the clips in the exported FBX; it was tested with Generic.

The FBX with *_LOD levels and fitted weights works anywhere; the switch heights and the measurement in the engine are for Unity.

No.