I don’t think there is any feature or workaround for that, just yesterday i was re-exporting some assets from 3dsmax some new ones and some old ones and the exact thing that you mention in point 2 happens that the mesh in Viewport disappears but are present in Outliner.
- Now what you can do is delete the Datasmith actors from your scene and drag and drop manually
your datasmith scene and just reset the transfrom and rotation to get the same position as you would in your 3dsmax file.
- After this you can go to delete assets not in level an delete them
Note :- I did not try the second point which is delete assets from level as i didn’t need to but i think this might solve your problem.