You might want to contact the vendor (Software Toolbox) directly. They may be able to explain the effects you are seeing with line width.
I took a look at this. We had plans to upgrade to Toolbox 2.5 in PB 3.3, but scrapped that plan due to multiple issues in that version, so PB still uses Toolbox 2.0, like you are trying.
Symbol.LineStyle is a PB property that has nothing to do with the Toolbox and probably has no effect on those symbols. As far as the TBSymbol class, the only formatting options I see are FillColor and FillStyle.
You can hook the ThisDisplay.AfterSave event and then get the file that was saved using ThisDisplay.Path and then modify the saved file.
Thank you for your response!