SIN-DPS is already booked so there is no changing that. For such a short ride, I don't care too much about comfort. I'm sure I'll be able to fall asleep after flying so many hours.
Coming back, I am not flying to DPS-SIN, I am making other stops and will be in SIN for a full day prior. It sounds like I may be able to use online checkin on UA and carry my bag to get into airside earlier