Does anyone know for sure what the order should be?
1.) W is W is W and that you should be able to SDC directly in to any open W seat, bypassing the upgrade list of people with main cabin fares on the W upgrade list (similar to how purchased FC tickets work where you can SDC in to any F seat, bypassing elites on the F list).
OR
2.) SDC for W relies on your underlying fare basis code and you can only SDC if your underlying fare basis is available (ie T,X,V) where if your cheap underlying fare isn't available, you have to SDS and therefore wait until T-15 at which point the main cabin to W upgrades have already been processed?
My experience (n=2) has been option 2.) both times, IE I've gotten a middle in the back despite being on a paid W fare.