Quote - Each body part can define a movement for a border vertex, but as each body part has its own geometry, these movement do not collide. But for preview and rendering - which body parts wins to define the x,y,z coordinates of the point that is used by the rendering engine?
My assumption was only one body part can define the morphed geometry.

This is what I have always assumed happens, though I have no special knowledge here, and may be wrong. The deformations (bending) at the joints is controlled by the join parameters (JPs). In the cr2 the JPs are the channels labelled "joint", "twist", and "smoothScale". I don't think Poser does specifically do anything to prevent the vertices from colliding. Rather the fact that they don't usually collide is a consequence of the sort of deformations, and restrictions on movement, introduced by the JPs, but the vertices can and sometimes do collide.

Quote - I am speculating that the figures are welded in the UI, and some vertices are not taking part in the surface any more because of the welding. But I have no idea how to make sure :)

At a welded seam, the vertices in the child will respect the position of the vertices in the parent, so that if the parent is morphed, the vertex positions in the child will be identical with those in the parent at the seam. A morph in the child will not affect the shape of the parent, or the position of it's own vertices at the welded seam.