I would recommend using/looking at Amazon SQS. It seems to all (or atleast most) of your use cases and is managed so you won't need to deal with yet another system. It is strictly a message queue so you would still need to do some work to get things wired up but it should be straight forward.

Another mention, Amazon Simple Workflow is also something you may want to look at. I haven't used it in any project but seems like it could, sort-of work here. (not particularly sure though).