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.
Hey For weapons I bake my hi onto my low with smoothing groups and UV splits along those lines etc etc. I go from max to xnormal through the xnormal SBM exporter. When I proceed to create a cage, I just grab my low poly, duplicate and push with a small value of say 0.5, and tweak some more in areas that still show the high…
Ah okay, so export the cage through the project modifier instead of exporting a duplicated push mesh. Btw your (or your teammates, not sure exactly who) Brink weapon bakes are some of my favorites ever, very clean work.