Regarding role based submission, that's something, I will be adding as one of the features.
I am currently working on an "auto-submit" feature, which is the first step towards doing a role based submission.
In a "role based" submission env, the system will create assignments for all the users who are the given role. The first person who claims the assignment will be able to work on it and move it to the next step.
Editing an already published page - That's another feature which I will be working on. I haven't fully thought through the process, but the general idea is to create a new/alt version of the post/page, change its status to draft and submit this version through the workflow.
At the time of publish, this new alt revision will replace the original post. So essentially the old post will be visible to the end user until someone decides to publish this new alt revision which will basically override the old post.
Hope this makes sense. Although, at this point of time, these features are not available in the plugin.