It's concerning and disappointing to learn that GWR staff are taking the view that they can invent conditions under 14.1 and thus make it so that 14.3 doesn't apply.
An inboundary Travelcard and a boundary zone ticket go together like a horse and carriage - they not only don't, but couldn't possibly, have conditions attached to them precluding their use in conjunction with the other.
Are they perhaps upset because it is possible to buy an undiscounted off-peak Newbury to BZ6 day return and a Network Railcard-discounted inboundary 1-6 day Travelcard, depart before 10am (on trains timed to arrive at Paddington after 09:52), and use it in conjunction with a Network Railcard-discounted inboundary Travelcard, as long as the train doesn't pass the boundary of zone 6 before 10am? I can see that they might not like that and might prefer it if people overpaid for an undiscounted ouboundary day Travelcard if they wanted to depart before 10am.
You are perfectly within your rights, however, if you want to begin your journey before 10am, only to forgo your Network Railcard discount in relation to the ticket(s) you are using before 10am, as long as you are complying with all the restrictions on the tickets. Whether GWR staff like it is immaterial to the question of validity.
It can, in my experience, be extremely difficult to buy a Network Railcard-discounted ticket at a ticket office before 10am even where it is obvious that you have no intention and, in fact, no possible way of travelling on that ticket before 10am. For example, when holding a 7-day inboundary Travelcard season ticket, I have tried at ticket offices in South East London to buy a boundary zone 6 to Maidenhead at about 09:20 and been refused. This was in the days before you could buy boundary zone tickets from ticket machines, and before the Elizabeth Line made it possible to reach Paddington in under an hour, and I didn't at that time know how to get them online. So the alternative to buying the ticket at my departure station was to queue up at London Bridge or Paddington, which could take a long time. There was absolutely no reasoning with them, though. Can't sell an off-peak before 09:30, can't sell a Network Railcard-discounted ticket before 10am, and that was that.
Maybe that's part of the problem? They know the TVMs are programmed not to sell these tickets before certain times, and they don't want to be in trouble for letting people get around that restriction by going to them? It sounds like the rule has been made up at a fairly high level within GWR, though. I would strongly encourage you to make a complaint - you are right and they are wrong.