Desert Traveller, Dont be stressful too much. SQ normally will release the seat 1 week before departure. They have management reviewing team that control the seat assignment or load factor. I always get clear on SQ flights no matter how busy is the flight. At least 1 week before departure or sometimes 2 weeks before hand.
Good Luck with your waitlisted status. PPS means nothing, I always using my UA MP1k status for SQ waitlisted but clear all the way through. QPP status only for back up.