It’s definitely quite a confusing issue and I can see why staff at national rail stations would also give mixed information as to sell an AAA 7-9 ticket they’d have to change their station to one in zones 7-9.
Just to clarify, if I bought online a TOD Travelcard from Amersham to Zones 1-6 AAA 7-9 I’d be able to pick it up from Slade Green, travel to Amersham, then go to Chesham, back to Epsom and then go to Dartford and board a 428 back to Slade Green without my ticket being eaten at the barriers (particularly Amersham)
I would normally use Oyster but with £5 off a ticket with Uber making the paper ticket £12 for the day it’s a bargain (and no max journey times to worry about as a bonus)
Chris