1) yes, position is media file number and record. scanner will rebuild (merge) what you are scanning unless already there.
3) It will be populated
4) you can use -m, but then only media records will be populated. Some backups, like RMAN, to be restored require index too.
5) if you have index available correct. As for tapes, mark one drive as read only, make sure autoeject is not set on and load tape with no mount option and run scanner.
6) correct (unless you are referring to full status based on native uncompressed value - in that case tape might still be appendable)
7) you can't unmount volume in use.
😎 you can do whatever you want.