Yes. It’s common. I don’t think it’s a bad abbreviation if parsed, ‘8v… b’
Sorry, my bad… I meant to type -8 to delete the previous eight bars etc.
Duration, yes. As with ‘dim.’, until a further dynamic marking or bars rest etc. It is a ‘direction of dynamics’ as ‘l’istesso tempo’ is a direction of speed.
It would be on a case by case basis. Some are obvious, others need serious flagging. These really are useful for studio or sight-reading work.