Thank you for your interesting study! Your results match my expectations, except for column 3:
MagWeb wrote:Doing a fusion (without filling holes) remeshes the involved surfaces to be one. You might expect such a target (column 3) as being handled the same way as we saw before in case of grouped meshes. But it isn't so. DAVID alignes the souce perfectly to the bigger part and ignores the smaller as if there were two partial meshes as long as the halfs'-surfaces are not connected.
Indeed, in this case I "expect such a target (column 3) as being handled the same way as we saw before in case of grouped meshes". You said that this happens "as long as halfs'-surfaces are not connected". That's surprising, since the triangle connectivity is only used for calculating the surface normals and for computing the mesh boundary. So triangles play only a minor role during alignment. Therfore, I think it is questionable whether one can generalize the result in column 3 to other cases. However, an aligment with a fusion result should be performed only in rare exceptional cases.