Late august? Don't worry, it'll clear. The shuttles are high load planes, and it's often overbooked way it advance but almost always cleared, especially if you are already on the waitlist.

The "problem" with the 1655 flight is that the plane is an airbus, and the shuttle is a 2 class service. That means they seat H class passengers in F seats. The J seats are mostly assigned to PPS members. Since there aren't many F seats, it fills up fast.

As for *G getting you headway with SQ.... I don't think it will happen. PPS members, especially solitaires, are handled as priority over all else. No harm trying though. You could also opt to fly MH metal, I've heard good things about their service.

As for the SIN-CPH segment, no reason why you can't get a preassigned seat as an H, in fact, all biz class seats are preassigned, usually your TA does that. Again, PPS status takes precendence - all PPS members get to choose their preferred seats on each SQ plane configuration and SQ takes it pretty seriously.