Should I use the Workbench Access module to implement the workflow we have in mind?
The Workbench Access module seems to only partially cover your needs. 
If it was up to me to implement a solution for the challenge you're trying to solve, I'd go for another approach, as further detailed below.

Have a look at my answers to either of these questions:


How to implement a multi step approval with webforms/entityforms based on form components?
How can I allow anonymous visitors to submit content?


These answers explain how you can implement a workflow that is pretty (though not exactly) similar to what you are looking for.

To do so, it "only" requires you to use these modules (and regular content types):


Rules.
Node Convert.
Flag.
Content Access.


And by using such approach, it does not require modules like "Webform" or "Workbench" (modules that are typically involved for these kinds of workfows).