“Mixed cabin awards are not bookable online. You must call to book a mixed cabin award by phone but note, the award will price at the higher class of service for the entire route on a mixed cabin award. And don’t forget to ask them to honor the 15% online discount (because you were unable to book it online).”
So it seems you are right. You will be charged First Class all the way 75,000 miles (after online discount 63,750)
You should try calling again and if you get the same response, escalate and ask to speak to a supervisor/manager.
After several calls, a lady knew what I really want and she sent me an itinerary but she said I need to wait for her to ask the manager for 63,750 instead of 80k plus. I started with asking how much does it cost from Sydney to Ho Chi Minh and she said 63,750. I told her my flight time and date, and I tried to ask if I can add a stop over at Singapore for $100, she said yes. But later she told me again the price would be 80k plus but she asked me to wait for that so she can ask her manager for cheaper miles and call me back.
Hmm, in the meantime, Im not sure if you have experienced the suite from Sydney to Singapore? They have SQ 222 and 232. The 232 which is the afternoon almost has waitlist for most of the day instead of SQ 222. So im wondering is there any difference on afternoon/ night flight in SQ for example catering? Both are on A380.