The multi-city booking on the QF website is one of it's more *problematic* features. I use it often because of my flight requirements:

- I find the flights the booking engine automatically selects tend to be more expensive and selecting other flights usually brings the overall cost down - brought the cost down 35% on my most recent booking
- The initial flights often don't actually match what you requested. No matter how specific I am about wanting an intra-Europe flight to land at LHR, the automatic selection will usually be a more expensive flight landing at LGW
- I've never noticed that changing more than one of the legs causes the error message. I tend to find that if you repeatedly search and then changing to the flights that you want, on about the third or fourth go you stop getting the error message and it takes you through to the next screen to complete the booking.

On the SIN stopover, you will deplane and wait in the gate area or pop into the lounge, although this is generally short - within 45 minutes you are usually going back through security screening at the gate for the onward flight to SYD.