Availability on SK flights usually has this christmas tree effect, I was eyeballing a MIA ticket for the Mrs for May which has become available, unavailable, different partners, different cost, no C, no Y and back to random mixes varying at least daily if not every few hours.

Overbooking is hardly a concern unless you have a very hard cut-off time for which you need to be in PVG. Normally they offer vouchers during overbooking to rebook passengers and there is generally more interest than need.