When creating remote BLOB store is "RBSFilestreamFile" always the name of the file to be added to the FILEGROUP?
The filename and filegroup name are not fixed.

SharePoint will use the same name when creating the databases.  However there's nothing that would prevent someone from changing those names to something else if they desired.  I'd query for them from the database backup using RESTORE FILELISTONLY then create the RESTORE DATABASE Command based off that information.