Hello Lancet,
As for your first three questions, I need to contact CDV US, if I still can, but let me have a look for you. As a developer we don’t manage the presence of the DLC on Xboxlive, it’s CDV US that took care of that.
4: Are PC players still able to purchase or otherwise obtain these sets, or has the release of DKS lead to their removal as well?
Those sets are in The Dragon Knight Saga. Atlus will have also unique item sets that are distributed with the US version (only for US Xbox360).
All the best,
Lynn
