Personally, I do not think that you have allocated anywhere near enough time in Siem Reap - Angkor.
Angkor is a region, a large region, not a singular point.
Airport check in counters at Siem Reap airport - REP - open exactly two hours before official flight departure time and close circa 20 to 25 minutes before flight departure time. Also, airport security checking at REP can be slow.
Only you know the financial costs and personal disruption that being refused a boarding pass or missing a flight and any connecting flights etc would entail.
Best regards. Ian/Angkorlad
Obviously you’re coming to Siem Reap to visit Angkor Wat so you must have some interest in ancient temple sites. Therefore you’re doing yourself a disservice to rush around in only one day. There are a great many fascinating sites to visit both throughout the archeological Park, also beyond into the countryside. Siem Reap itself in a great place to explore, so before you finalise your itinerary it would be worthwhile doing some research on the area. It’s probably one of, if not the most interesting, vibrant, varied and fascinating cities in South East Asia. You wouldn’t even begin to touch the surface in 1 or 2 days.
another vote for 2 or more if you can juggle your plans - you wont regret it.
Leaving town/your hotel at 5.45-6pm should give you plenty of time.
Be prepared to talk about a return visit to see more as early as the airport departure lounge.
For sure you will need at least two days, one day to explore the temples and another to explore the town and other attractions. I cycled around the temple complex for a whole day ad felt it was enough for me, but many visitors get a 3 day pass .
Spent 3nts in SR in March 2018 and we so wished we had stayed there longer. 1 night is nowhere near long enough.