Scene View - Left-Hand XYZ Coordinates

May this change the mind-set in any computations from those working from the right-hand world to those in the left-hand world?

Basic mathematical operations like computing the cross product, building and applying rotations, and so on will all be the same regardless of handedness (although the visual effect may differ). View and projection matrices are generally constructed differently depending on handedness, but you usually won't be dealing with those directly in Unity.

Also, if importing assets from 3D-CG-package that uses right-hand rule, how is this resolved in Unity?

There's a variety of issues that can come up when importing data built in and/or intended for use with a right-handed coordinate system into a left-handed system or vice versa. I don't have that much Unity-specific experience in this area, but some of the changes that sometimes have to be made include changing triangle winding, mirroring a mesh along a cardinal axis, or 'mirroring' rotations. (Note that a rotation in quaternion form can be mirrored along an axis by negating the real part of the quaternion, and negating the imaginary part corresponding to the axis along which to mirror.)