The ray cast algorithm is expensive to perform at higher resolutions with complex scenes. I would recommend the following options to optimize:

• If you have frame range querying enabled, increase the frame step (do not worry if your active camera changes, those frames are always added, even if they are not on the frame interval)
  
• Reduce the number of bounces
  
• Increase the tile size
• Reduce the number of objects selected. Also, instances, meshes, metaballs, and font object types are much faster to evaluate compared to other object types, so removing other object types from your selection will improve performance.

This is likely a case where the resolution scale is too low. Try increasing the scale in small increments.

The undo functionality works for all nView operators. But if you went beyond your undo size or do not want to undo other operations, the clean panel will allow you to show all objects and collections, as well as remove visibility attribute keyframes.

This addon is independent of the render engine. It casts rays internally and applies changes at the scene level. So you can use whatever render engine you want!

No, it does not. nView does not automatically run as you navigate your 3D scene. Operations are applied to selected objects.

The ray cast method grabs faces based on the current evaluated scene, which means modifiers that add or remove geometry will offset or hide geometry that's otherwise visible (see known issue in Blender repo - #72113). The ray cast method may see faces whose index may not exist in the original mesh or have been offset by a modifier.

Try disabling modifiers in edit mode. Unfortunately this also affects the mask operator when using the ray cast method. This is an ongoing bug that's being looked into for an appropriate solution while keeping current behavior.

This is a known issue in that the parameters aren't separated. This is being looked into.