Changi
Cooda,
The 16h layover is something NOBODY in their right mind wants or needs - however nice Changi is.
The hotel can indeed be rented for 8h shifts - and is not very cheap. It also has no windows in any rooms !!
Personal recommendation would be - to re-route via HKG or KUL and make certain that the layovers are shorter. Both airports are easier than Changi, unless you've collected so many airports that another one won't phase you. You might even consider BKK with a 3-4h layover if you can get it, though ability to communicate and understand may be more limited there.
MAN-SIN is around 12-13h a 16h layover and then a 7-8h to Oz and the 4-5h needed to get to the airport and do the check-in (2h prior). That's over 40h on the trot.... :o