Several things that you are misunderstanding:
1. The Read-only/Read-Write settings in your project file do not apply to Content Center; they apply to your Styles Library, and Material/Appearance libraries
2. All Content Center parts that are placed as Standard are not editable as long as the active project file has that location designated as the Content Center destination; that is, the project file setting for Content Center Files is the destination for Content Center files (placed as Standard), and that location is read-only
3. Items that are placed in an assembly have no memory of what project was active when they were placed, so there is no need to worry about accidentally using the wrong project, with one exception: the destination folder for Standard Content Center parts might be different, so parts could end up in a different folder from the intended one.
All this is by design, with the assumption that standard fasteners are not meant to be edited.
Two ways to get around this restriction:
1. Set up another project that does not have that location set as the Content Center Files location; make this project active, then open the assembly and edit your BOM as you please
2. Instead of placing your Content Center parts as Standard, place as Custom; this will then ask for filename and path, and you can save it to a folder that is not treated as read-only; note that Auto-drop assumes placing Standard, so cannot be used with this technique

Hope this helps,
Sam B
Inventor Pro 2019.0.1 | Windows 7 SP1
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