Hello Dieter,
1) What is the problem with meshing of the external dense clouds? If all you have is the dense cloud, then just use Import Points, then Reset Region, then Build Mesh with disabled or extrapolated option.
2) Have just checke the TLS export to version 1.5.1 Viewer and it has been properly displayed, which issues you are observing?
3) If you are speaking about the dense cloud generation, then you can re-use the depth maps created during the new meshing option, providing that the depth maps have been stored in the project (Keep Depth Maps option has been enabled in the Advanced preferences tab).
4) Currently it is not possible. As a workaround, however, you can try to export the mesh in OBJ format, for example, and remove the polygons from the file, keeping only vertices that will be then treated as points of the point cloud.