Hello Chen,
you're right; retrieving the workflow context in the event microflow of "User Task Started" requires both the workflow and the workflow object. The workflow object is not available at this time.
However, going back to your request to display context details in the inbox, why don't you use the following solution:
On the page "TaskInbox_xxxx", add an additional column that shows a snippet. That snippet is a dataview of your context entity.