If you want to only push a portion of the mesh, make a selection before adding the Push modifier and that modifier will only act on the selection. You don't need to unweld anything.
I've been wondering is it possible to create a stamp that has a push and pull effect, instead of just doing one direction. I notice that wherever the stamp is black it keeps the surface flat. But in zbrush i believe black pushes down and white pushes up? I find it kinda strange that mudbox is different.
Make a box, convert to poly, make some cuts, push vertices to closer represent final form. Repeat cuts and moves until you satisfied with how it looks. Sculpt over it for fine detail. Bake to lowpoly.
make sure you've set the modifier's luminance mid point to 0.5 ... if it's 0.0 then it will only push out, if it's 1.0 then it will only push in. 0.5 will use mid grey as mid height.