If you do want the Yotel/Aerotel/(Ambassador by Harilela), do make sure to book it prior to getting there.
This ensures they have a booking for you.
Those rooms are in high demand, and sometimes they do not have any open spots for "turn ups".
I have done 7 hours, but only during the day up to evening tho, few years ago.
I don't think they kick people out, (eg like MEL/SYD/PER secure zones, where they close off the secure zones for cleaning after the last int flight has dep), so you could, if you want to save money, is just to sit in a certain spot for the 9 hrs.
Changi would be safe, but (maybe) still best to have that neck wallet thing that holds your passport and cards, and $, so you don't leave things, behind, accidentally, worn under your shirt.
Its hot and humid over there, on the outside, but the airport is aircond so could be cool, so maybe a light puffer jacket or a 2nd shirt might do to keep a bit warm.
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GST goes up to 9%, plus service (tax), for all hotels in SIN, so if you want hotel, or shower, have to pay a bit more from tomorrow/in a few hours time, anyway.