Looks really good. Moreover good to follow standard, especially while bearing in mind v5 that will be based on WS-HT.
If it comes to your question about distinguishing between manual cancellation and timer I would say that it can be important from business point of view.
Since expiration usually means some kind of escalation meaning task (or information that should be gathered within this step) are important for the process and cancellation could be done just to skip certain types of steps in the process and by doing that process can have inconsistent state - no one has completed the task and it was not escalated so some of the information can be missing. Especially that signal can be made by providing any transition from all available.
Perhaps an alternative to have different states/cancellation methods would be to add a task comment while signaling in skip mode?! At least some information why the task was skipped.