I got great help from an earlier question regarding Indented BOMs. Happy to report that is working well so far. OurBOMs are now viewable in Indented BOM format, and the Parts Only function tabulates & collapses the Indented structure as hoped.
All is well in BOM Land for the mechanical hierarchies created in SolidWorks. Unfortunately we have thousands of parts embedded in Excel files, which are outputs from our PCB design group, who generate PC Boards & PC Assemblies in Altium. Down the road - when time & funds permit - we will consider better integration with Altium. In the meantime we have decided to manage the PC drawings (a Smart pdf output) & PC BOMs (Excel) in EPDM to maintain a centralized repository/enterprise process. Ideally the PC BOMs - captured in separate Parts List Excel files - would be part of the EPDM Indented BOM structure, without having to create SW parts for all the PC parts...and generally with as little post processing work as possible for those loading the files, and generating the BOM outputs.