One of your 3D sketch gives error when I tried to open your file.
I didn't go in detail, but the way you create the parting line and parting surface, seems to work.
I rolled back your model to just after the Body Delete & then used the split command using the large flat face as the trim tool. The part was cut into two solid bodies which can then continued to be edited, or if you select to Auto Name the resulting cut bodies, two new parts are created & opened which can be modified. Although I don't think these changes are propagated back to the original part but someone might correct me on that.
Thanks Kevin that's exactly what I needed.
Yes it does link to the original, and through to an assembly.