If I were you, I'd just book a hotel room in downtown S'pore and not worry about it.
If you go to the airport in Portland and they let you check your bags all the way through to MDC great. If they don't it's still not a big deal. Either way you can zoom through immigrations and customs (bags or no bags) and be in downtown S'pore before 2 AM.
Coming from Portland it will be the equivalent of 11 AM for your body so you probably won't be too sleepy anyways. If you're into Southern Chinese/Malay food you could catch a late night/early morning snack at one of the 24 hour hawker centers for a true Singaporean experience (try Lau Pa Sat or Newton Circle - any cab driver can take you there) then back to your hotel for some zzz's before heading out to Changi in the morning for your Silk Air flight.
Taxi drivers in S'pore all accept credit cards so if you don't go to a hawker center you wouldn't even need to get any S'porean dollars.
By the way - Silk Air gives you the option to print your boarding pass at home 48 hours before departure - that might be enough time for you to print it out in Portland before you leave.
Most locals avoid Newton circus. The food there is grossly overpriced and there is a history of touting in that hawker center. Lao Pasat is ok, but I would recommend Makan Sutra (Glutton's Bay) for its location. It is right beside the esplanade. The night view there is pretty good. Food is decent and price alright. The good thing is all the prices are clearly displayed so you won't get cheated.