Yes, the lack of NR trains is probably why it won’t sell you the ticket. You could try Brentwood as the origin instead, or Cheshunt, or maybe Watford High Street. Any zone 7-9 National Rail station should work, except Dartford.
That’s very strange. The route “AAA LDN ZONE 7-9” makes the ticket a day travelcard valid throughout zones 1-9. Normally the off-peak version of that is only valid after 0930. It’s possible that 0915 is a local easement for Watford High Street. I don’t think the gates at Chalfont will accept it before 0930.
Beyond that there is no need to go to Watford High Street at any time. It’s just a 1-9 travelcard.