What are my obligations towards my adviser after the defense, revisions, and all forms signing is done?
Other cultures may have different approaches to the ethics of this, but I think it's pretty out of line for the US.

I would recommend standing by your commitment to help organize the workshop, but I would not do your former advisor's reviewing for him. You've graduated, you're a fully-fledged member of the doctoral community. 

Pete's comment is pretty spot on. Be as nice and cordial as possible, but be firm in your refusal to do his work for him.