There are a couple of different things you're bringing up here.
If you want workflow in the sense of a content production and aopproval procedure, then you have to get modules that have that functionality built in. The DNN framework itself doesn't provide that (although there has been some talk about providing something like it in future, presumably to better deal with what I'm about to detail below).
One of the things that really confuses is that the DNN framework really has a couple of content management scenarios. In one scenario, the content management role exists at page-edit level. At this level the user can create new pages, and add and remove modules from them. They have full access to all page and module settings, and all module functionality. The only way and admin user can control things at this level is to restrict the page editor to a specific branch of pages. As well, the host user can control which modules will be available on the module dropdown list. However, this is per portal, not per user. The page editor role should be seen as a webmaster type of role because creating and removing entire pages and modules is about information architecture and design, not about content.
There is another level of permissions below page editor and this is the correct content manager role. This role is one that is granted edit permissions to selected modules. This role does not allow the user to add or remove modules, but it does give them access to content management functionality within the module. This is really how you should be controlling content authoring security in a DNN portal system with multiple authors. The content manager role with module edit rights is strictly there to produce and process content.
As I've mentioned elsewhere... The content manager role fails when module developers don't understand its purpose and place content management funtions out of reach inside the module settings page. You have to check third party modules for all required functionality including this.
In your posts, it sounds like your content managers are going to be page editors. Consider carefully whether this is really needed.
I suggest doing some tests with DNN. Create a portal, create the roles I've described, set up some test pages and modules, and try it out. DNN has many strengths but built-in structured and secured workflow is not one of them and you may find you need something else. But you need to fully test it yourself to determine that.
Rob