Please accept my apologies for asking what may be naďve questions, but a search of the forum and FAQ didn’t turn up clear answers.

I am looking to book a LONE4 RTW fare (with a complicated 16-segment itinerary). For the first flight, SYD-HKG, there is currently no viable choice other than using CX metal. But for both mileage accrual and customer service reasons, I would much prefer to book that under the AY codeshare flight number, and have the ticket issued by AY. The AY chat agent indicated that they can’t do that, that they can only handle ticketing if they operate the first flight, and the ticket must be issued by that first operating carrier. But that doesn’t really make any sense, because if that were the case, why would AY even publish fares for a LONE4 ticket ex Australia, when they have never operated to Australia at all?

Do you have any suggestions as to a way around this, and/or know of a good booking site which might allow me to construct this itinerary using the codeshare flights? I know that agents can do that manually, since I have done that in the past. But then have to deal with the agent’s limited hours and imposed service fees for flight changes. And the OneWorld tool doesn't ever seem to show any of the codeshares, even when ExpertFlyer does show them, and with L class availability.

Also, the OneWorld booking tool is acting like the following itinerary is not legal, having too many segments in Europe/Middle East. But I think I am counting them up and it is correct:

HKG-HEL
HEL-LPA
LPA-EUN
EUN-CMN-BCN
BCN-MIA

I see four flights within Europe/ME and stopovers in four cities in Europe/ME. What am I missing here?

Finally, does the ‘preferred airline’ box of the booking tool actually affect anything? It didn’t seem to in my experience.

Thanks for your thoughts.

In the past (more than 2 years ago) i had my xONES issued by a carrier different from the first. Back then I was always using a (real) TA, since i could just email him for changes etc.
I was worth every penny he was charging, since I was usually changing my routing midstream once or twice and also nested other tickets with the RTW. He managed to add them to the same reservation and checking luggage thru on differen tickets was never a problem.

I am still reluctant with all the changes of countries which you can enter to use RTWs.

You could book through a TA, have them tkt on AY, then just deal with AY after the first flight has been taken. You don't have to make subsequent changes through the TA.

Or just call AY to book - RTW trained agents are less likely to turn you away - the chat agent may have been speaking generally (unless they were RTW trained?). Call and try to book with AY, at least try until you've been turned down a few times.

As JAXBA notes,
OP, another snag you might face is that some airlines don't want to ticket an RTW unless they take you over at least one of the oceans. If you go with AY, that might limit your options on the TATL sector. Or maybe not. Or maybe HEL (soon ARN) to XXX in North America can be made to fit into your itinerary.

The issue of booking the CX flight as an AY code is separate from which airline tickets the trip, although many online systems automatically ticket using the carrier whose code is on the first flight (not the carrier operating it), and most travel agents by default do so as well, it's possible to override that and ticket using a different airline.

In general, people booking OneWorld RTW fares typically prefer to ticket using either a carrier that makes it easy to make changes and reissue the ticket, or a carrier that prices with fewer carrier surcharges (e.g., BA tends to add its own carrier surcharges).

The OneWorld automated tool will always try and book using the carrier whose code is on the first flight, but it may not show codeshares.

Pre-Pandemic time, I had first flight on QR but QR did not want to (or was not able to) ticket. I always (on two separate DONEx) called AA and AA ticketed them with ease. Later, when I had to make changes, I called AA to get them done. Never a problem.
I am NOT conversant with the new rules or any updates there off (since Dec 2019), but it looks like you exited Europe (HEL-LPA) and then returned to Europe (EUN-BCN). That might be a red flag. I don't know if LPA, EUN are considered as being the ME. Others might chime in to help.

Both LPA and EUN are in Europe. Canary Islands are southern most part of Spain. Morocco for OW purposes is part of Europe as shown below from the OWE rules:

The reference to "RTW must be ticketed by first operating carrier" is with respect to those itineraries booked/built using the oneworld online tool. I book AONEn trips with the AA RTW desk all the time with CX as my first carrier ex-HND.

Good to know. Thanks!