One CRITICAL restriction on most SQ RTW's (with exception of higher price *Alliance RTW's) -- you cannot backtrack -- very few exceptions.
For example I tried to include Sao Paolo Brazil in a SQ/UA RTW... no problems BKK-SIN-SYD-SFO-ORD-GRU ... then problem GRU-IAD is considered backtrack ... rest of itin OK IAD-FRA-SIN-BKK ... pity was a goof fare in Business class (around $3.4K)