Hate to bring you bad news but ALTER DATABASE command that you referenced is supposed (and working as expected) only in 8i. If you had to use it on 8.0.x, you may be getting more problems in the future... Also, us7ascii has the 8th bit anyway , it's just not recognised so you are fine by letting export default to us7ascii even if sys.props$ states it's we8iso8859p1...
My advice - if you have an NLS question or problem - post it to Oracle Matalink forum where you will get a more accurate answer - no offence to this forum, it's just documentation on NLS is still not that good... Please trust me on that! Unless of course if you want to corrupt your database :-)