I try to always issue my tickets through AA even if they are not one of the carriers.
If you want to do a date change and ring up QF (maybe BA and CX as well) they will charge you upwards of $50 above and beyond any change penalty for the inconvenience. AA only charge whatever is indicated in the fare rules - in the case of an xONEx $0

Hmmm I assume for Qantas that's the phone processing fee they charge for doing anything. I guess I'll go with American. Will the ticket be issued by AA even if I fly AA codeshare on BA metal for the first segment?

Also, when I called the AA RTW desk, the lady told me the cancellation fee was 5% of the fare (for business) + the segments already flown. Wouldn't it be cheaper if I just let the ticket run out in the end? (If I do want to cancel that is)

Yes. It will be issued by AA.

Yes - 5% is correct (see the rules here, if you haven't seen them already - rule 16 specifies cancellation fees)
I can't say whether just not showing up for any remaining segments would be cheaper than cancelling. But I expect that if you have only a few segments left, that would be the case

I agree. Instead of flying to LHR on your separate ticket, you might want to consider Riyadh (RUH) which, depending on exchange rates, is normally the cheapest place to begin a RTW. It can save thousands over the same ticket begun at LHR. Make the first and final legs RUH-LHR/LHR-RUH on BA, then continue as you have it making a couple of changes because of the two extra segments.

I agree, but IMHO it might be better to consider AMM instead of RUH
- no visa problems
- probably easier to get to from SYD than RUH
- good place to spend a few days (Mrs P and I hired a car from the airport back in '09 - Dead Sea, Crusader castle, Petra, Roman ruins at Jerash)
- a DONE4 ex-AMM only costs ~USD400 more than ex-RUH
- can start with an RJ flight AMM-LHR ; thus avoiding starting with BA and paying its high fuel fines

Of course, if the OP really wants to get the cheapest DONE4 - then s/he should start from KRT!!

Thanks for all the help guys. So at the moment, my plan looks a little like this.

FRA-xDFW-SFO-LAS-LAX-YYZ-ORD-JFK-SYD-NRT-BKK-HKG-SIN-LHR-DXB-xLHR-FRA (then purchase a one-way back to LHR)

I do have a few problems though. It lets me fly DXB-FRA via LHR no problem, (in effect violating both the 2 stopovers in Origin rule and also the flying return between the same city pair rule), but it counts DFW-SFO as 1st of the 6 NA segments?

Reason I ask is I am trying to book one more segment into NA. I know I can buy extra segments at 200GBP but am just trying to get clarification here. Perhaps there is an email address for RTW Desk at AA? I assume if I do add a segment I have to email them to get a quote.

Also, peaking of the AA RTW desk, the lady who answered last time told me I could not book an open-ended ticket even though theres a section in the rules that state (for travel starting in TC2/3): "Reservations for the first international flight and all preceding flights must be made prior to departure. Subsequent segments may be open-dated". Was she just wrong and I should have argued it with her?

That routing doesn't violate any rules and counting DFW-SFO as NA 1/6 is quite correct. It isn't FRA-SFO. It is FRA-DFW-SFO.
1: FRA-DFW: 1st of 1 (intercontinental entry to North America without immediate transit & departure)
2: DFW-SFO: 1st of 6 (North America)
3: SFO-LAS: 2nd of 6 (North America)
4: LAS-LAX: 3rd of 6 (North America)
5: LAX-YYZ: 4th of 6 (North America/North America Transcon)
6: YYZ-ORD: 5th of 6 (North America)
7: ORD-JFK: 6th of 6 (North America)
8: JFK-SYD: intercontinental
9: SYD-NRT: 1st of 1 (intercontinental entry to Asia without immediate transit & departure)
10: NRT-BKK: 1st of 4 (Asia)
11: BKK-HKG: 2nd of 4 (Asia)
12: HKG-SIN: 3rd of 4 (Asia)
13: SIN-LHR: intercontinental
14: LHR-DXB: 1st of 4 (Europe)
15: DXB-LHR: 2nd of 4 (Europe)
16: LHR-FRA: 3rd of 4 (Europe)

Your current route has 16 sectors. You can't add another one, paid extra or not, unless you drop one somewhere else.

They need to enter a date into the booking. It's a restriction due to etickets. You can change the date after ticketing, or try to get a paper ticket.

I was under the impression that I can buy up to two extra segments for each continent (other than origin) with no more than 20 segments in total for the entire ticket.

With the above route you're not violating any rules regarding stopovers in the continent of origin - you have two shown, LHR and DXB. The rule regarding city pairs is that you can't fly the same segment twice, e.g. LAX-DFW-LAX-DFW.

I presume you're also aware that SFO-LAS is via LAX (same flight number, two sectors) that starts next month, and is not necessarily a permanent thing. AA sometimes shows "direct" flights that actually stop en route. There is no nonstop Oneworld service between SFO and LAS.
Used to be that 20 was the maximum amount, counting supplemental segments. No more; 16 is now the magic number. You can't exceed 16 overall, full stop.

The only way the 16 sector limit can be overridden is if the airline changes something. Such as due to a cancellation.
eg, In 2008, I had booked a 16 sector LONE4 with NRT-MEL. QF canceled the
NRT-MEL flights after I ticketed, so rerouted me through SYD. Making it 17 sectors with NRT-SYD-MEL.
In 2009, I had booked a 16 sector LONE4 with a single flight number DEN(ORD)BOS. After ticketing, AA canned the flight and rerouted me via DFW, making DEN-BOS into DEN-DFW-BOS and upto 17 sectors.

If you are doing the LHR-DXB-LHR turn around purely for the miles/Status credits; might I suggest LHR-MCT-LHR instead?

Agree. But the itinerary is not really miles/status optimised anyway.

I am doing DXB because I want to visit there. With my current ticket, The destinations which are optional to me are NRT (been too many times recently) and HKG. HKG is only there so I can book a side trip to PVG easier.

How would you suggest I route to maximise miles/status? Assuming I want to keep all the locations that are currently in there except for NRT and HKG? That leaves two wild cards I am open to.

I am correct in assuming then that I can by extra segments within a continent as long as it doesn't go over 16 segments overall then?

Will it be cheaper for me to book by doing it through AA RTW desk (and paying in AUD) or should I just book it through the OneWorld planner?

Yes, but you can't add segments in the continent of origin, and any extra segments in North America can't include a second transcontinental nonstop.

If FRA-DFW (AA) is your first segment I don't think you need to use the RTW desk. Your ticket will be issued by AA and if you need support post-sale you can still deal with the RTW desk then.

I don't know if anyone has posted experience of using the online tool to buy additional segments. You could give it a shot and report back.

You can't buy extra segments using the online tool.