Has worked perfectly fine for me with several routings. Maybe a few things to be aware of:

• First segment has to be BA, IB, RJ - and the tool seems to prefer connecting first segments (i.e. CAI-LHR-XXX, CAI-MAD-XXX)
• Anything involving QR in later segments seems to be hit and miss - esp with the availability on EF actually being there
• Same as above with QF segments - especially longer ones
• Alaska routings are hit and miss - they seem to work better as part of a connection pair
• When a segment is only available in Y, often the tool will display a nonsense error like too many segments
• The "too many miles" is almost never actually because of miles, but because it's choking on some other rule violation

Hope these tips help!

Milecalc has that at 36K miles.

I was able to get to the final screen and price out CAI-DOH-JFK-LAX-DFW-JFK-SYD-DOH-DXB in the tool, but when I tried to pay, the tool sent up a message saying that the CAI-DOH-JFK legs weren't available. Upon closer inspection, QR had availability for those legs in D from most points of sale, but not ex-CAI. The AA codeshare on DOH-JFK was available but didn't show up in the tool. I instead booked a CAI-LHR-JFK start and that ticketed, although I had to call QF to give my credit card info a second time.

Also can't start ex-CAI fare with QR, so that's likely your issue.

Yup exactly, most people who want QR for the initial long haul will go via AMM on RJ to get it to work on the tool. You definitely ended up paying a lot more in YQ / YR by going on BA.

I didn't, actually. Pricing out CAI-AMM-JFK-etc was actually more expensive than CAI-LHR-JFK-etc.

I will also say that even if the online booking tool processes your itinerary and accepts payment, resulting in a PNR.. there is NO guarantee that your trip will actually ticket. I just went through that nightmare recently -- online tool "accepted" itinerary, but reservation never ticketed. Despite contacting many QF phone and Facebook agents, I kept getting a range of nonsense excuses ("You can only have 5 stopovers") or "I'll put it in the booking queue" but nothing happened. If the system doesn't automatically ticket your reservation, good luck finding a knowledgeable agent that can diagnose the problem AND fix the problem. If there's any consolation in the online tool route, it's that there is never usually a wait to speak with a QF phone agent. The FB support, although slightly more knowledgeable, was not consistently responsive in a timely manner.

I do think that's what happened with my QF booking from the RTW site. Once I called in and paid manually, then I got a receipt with an e-ticket number.

My last one QF sent me an email about 48 hrs after booking that my credit card didn't process so please call in. Did that, and ticketed within the hour.

I can manage to price DONE3 ex-CAI itins on the online tool at the price shown on EF. But whenever I try a DONE4 (which is what I really need), the price doubles (while it should only be a slight increase according to EF). Any idea what may be going on?

That's helpful, thanks.

What seems to have fixed it is clearing out my cookies, coning back as a guest and selecting QANTAS as the preferred airline before I put any cities in. Not sure which part(s) of that helped but itineraries I couldn't do before are working AOK now.

It threw the mis-labelled 34k distance error with CAI-MAD-DPS...GRU-HEL-DXB where-CAI-MAD-DPS...GRU-DOH worked fine so that's one thing it can't cope with. I may see if I can do a manual reissue later (would be more confident doing that with AA or BA or even JL/CX but I doubt any of them will touch it after QF issue).
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It sounds like you're adding in expensive segments in terms of YQ/surcharges. If you post what you're adding in someone might suggest which the culprits are and a possible alternative.

A price change is imminent I think, twice now I've seen a DONE3 base of 7,289 AUD crop up before it reverts to 3984 on an edit

Which flights and airlines are you booking as part of the DONE4 as opposed to the DONE3? Note the base pricing for the DONE3 on the pricing page (not including the taxes and carrier surcharges) and then get your DONE4 to the pricing page and see if the base pricing goes up dramatically. I would guess that most of the change is taxes/carrier surcharges on the DONE4.

I believe they are changing the prices, earlier I had 5.5k AUD, after changing the dates of departure, it become 8k+

No evidence of that. Still prices out lower for me.