So long as your second time through SIN is less than 24 hours, no rule is broken.

However ... your routing as given comes to 29 062 miles on the Great Circle Mapper. The GCM does not correspond exactly to the calculation that will be done by the rate desk of whichever airline issues your ticket, but it's a good rough-and-ready indicator. And what it indicates here is that you are slightly over the 29 000 mile cutoff point for the xRWSTAR1 fare. Thus, you will have to pay for 34 000 miles (xRWSTAR2) whether you use all of them or not.

Therefore, if you prefer to save money, you might want to tighten your route a bit (e.g., substituting SFO for LAX brings you under 29K on the GCM; if you need LA, you could easily go there on a separate ticket). Alternatively, you can fill out your itinerary to the full quota of 16 flights and 34K miles, thereby maximising your travel experience and your accumulation of Frequent Flyer points.

cheers,

Henry