Regarding my first point, I think that orientation can make sense. I don’t, however, think enough user feedback is given. This may be causing the disorientation.
Apple’s Reality Composer is a good example of feedback. When reaching the top of your model, the camera stops, preventing you from continuing to move. In Shapr3D there is no feedback at this point. With the absence of feedback, I do not expect the gesture motion to shift. The subsequent sudden change in motion is a surprise. Adding feedback would help make the implemented orientation more obvious.
With my second point, when working with a large assembly at a distance, I only want to rotate about the full assembly. Although rotating about a single part is useful, I tend to only do this when closer to the model. A gesture that separates a part’s Centre of mass with the Assembly’s Centre of mass could be useful. A ‘tap and hold’ on a part could come into play…although I know you already have plans to use it with the cube.
Another small gripe occurs when rotating the camera. When moving the camera close to the horizon, the grid density rapidly changes which is slightly irritating. Although the camera does get closer to the plane, the density does not maintain consistent with the reverse side of the plane which instantly changes to the highest density.