The travel agent does not interpret the rules. The airline does. Whatever the agent does, he/she still has to send the whole itinerary to the airline's rate desk which they are issuing the ticket on (usually the first carrier outbound) for approval. The airline rate desk will calculate the fare, the taxes and anything else to the agent. Who in turn will eventually be the one issuing the ticket... according to what they have been given.

So if the airline says no... then it's no. The TA must adhere to whatever the airline approves before the ticket is issued. So yes, it still boils down to whoever is pricing the ticket AND the interpretation of the rules.

Yes if it's a Oneworld ticket, say, that one may issue the ticket on whatever airline ticket stock. But then again, if you are departing from say SYD and your first outbound is QF... and you would prefer to have your ticket issued on AA... the TA can only do that if AA priced the itinerary.

AA can refuse to do so because there isn't any AA international flight segment on the itinerary, or if they are not the first outbound carrier, or because they just don't feel like it, (and they have refused to do so in the past), and even though the interpretation of the rules is to your advantage, the agent must adhere to whoever will price the fare and give the ok to issue on their ticketing stock.

I won't go into the horror stories that I have witnessed in the past when one TA issued a ticket on BA ticket document based on a fare priced by AA's rate desk because he preferred AA's interpretation of the rules and fares than BA's. Now that wasn't very professional of that TA to begin with! Because when the passenger tried to reissue the ticket, nothing matched on the ticket with the respective rate desks' quotes.