Here's my plan for you:
Day 1, SIN-REP flight
Days 2-4, Siem Reap. (Floating village in March dry season will be up on stilts rather than floating, ahem)
Day 5, morning bus to Battambang (3 hrs max), rest of day Battambang
Day 6, Battambang
Day 7, morning bus to Phnom Penh (5 hrs), rest of day PP
Day 8, PP
Day 9, PNH-SIN flight

If you had a couple of extra days, you could bus down to Kampot/Kep from PP then return. Don't cut a bus-flight connection too close though.

And wherever I use "bus" you can substitute "share taxi" or "private car" depending on your budget/time/convenience value matrix.