Reporting back, i managed to easily do the zoo, river safari and night zoo all in one day (actually just an afternoon and early evening),. Did a heritage tour at The Fullerton beforehand, and went to hawker centre in chinatown afterwards.
Monday seems like a good day to go. Entered zoo just before 2pm, saw everything i wanted including 3 shows, just gotta love the orangutan feeding, then watching a 2yo orangutan have a stand off with some otters released into their enclosure. Walked into river safari at 4:45pm, virtually no one there,zero waiting, guessing people with kids go in the morning, done by 6pm. A drink whilst waited for 7:15pm night safari.
Easily done, with no kids in tow.