You are correct on most accounts.  For the normal figures in Poser, each body part is a group of self contained polys and their associated vertexes.  Within the OBJ files, they are separate poly groups but are the same mesh.  When the OBJ is loaded in Poser, it breaks apart the mesh and duplicates the vertexes at the seam boundries.  It then re-welds the seams.

PhilC has a demonstration python script to demonstrate this:
http://www.philc.net/forum/viewtopic.php?t=3021

As for the morph questions, morphs move vertexes not polygons.  If you have a morph that moves a vertex in Part 1 to 0,0,1 and  a morph that moves it to 0,0,2 in Part 2, I believe that Poser will average the two positions for the vertex.  This will result in seam distortion.

For morphs that involve border vertexes, its best to weld the mesh and do the morph.  This will insure smooth seams.  You will then need to break apart the mesh and load each obj as a MT for the corresponding body part in poser.

For linking morphs, its simple as adding a few lines to the CR2:
            valueOpDeltaAdd
                Figure 1
                hip:1
                Thin
            deltaAddDelta 1.000000
In this case, this is from a bikini I made for K4.  This comes from the Thin morph on the thigh.  In this case, its linked to the Thin morph in the hip using a 1 to 1 ratio of movement.  The deltaAddDelta line defines the connection ratio.  For normal morphs you will typically use the 1:1 scenario so in the example above, the Thin morph is applied equally to both the thigh and the hip.  This is known as ERC or EasyPose.

If you want to form a curve in a tail or rope, each morph links to its parent using deltaAddDelta 2.00000 ratio, or a 2 to 1 movement. For each ONE increment of the parent, the child is applied at TWO increments.  Since each is a daisy chain from the prior, it will form a curl.  This would likely be applied to a twist, bend or scale parameter dial.