As relates to your procedure, RDS stomps all over materials, as you know. You can keep them, if, before you take the item into RDS, you change all 'usemtl' to 'g'. You must, of course then open each item as a single group in order to get at it. When you save the final mesh, you then reconvert the proper 'g' back to 'usemtl' - you may have to add back some real g group lines - if you have two g __ lines in sequence, the first one may have been removed of save.
But about your main problem - you can delete edge verts - no problems - you can internal weld verts - mapping distortions - BUT . edge weld verts -to other groups - mesh with different mapping - bad news.
What you should try - rather than weld, superimpose. select the two verts that you would weld and type in an identical location for both.
It is weld that is causing the problem.
There will be JP split seams maybe, and it may take some patching.

Oh, and I do not know what causes it or what will trigger it, but sometimes, RDS will flip normals. This can be really difficult to fix? but, there is somthing about Poser not wanting normals to begin with? It will do its own? DAZ wants 'vn' lines removed from obj files?
I have not experimented, but maybe Poser itself will put messed up normals right.