Unfortunately Sitefinity does not support permissions on per user basis. Also, I can see your point with not wanting 200 roles in the system.
I think you may be best off to turn on the workflow, and disable the store managers ability to publish pages. You could give this permission to the Regional Managers (or some other person for that matter). So even though store managers would have the ability to temporarily "mess up" pages of other stores, RegionalManager would easily notice this and would be able to take appropriate actions. Also, there would be no damage since changes could be rolled back and would never be on the actual site until published by Regional Manager.
If you are into development, you could even subscribe to the static Executed event of CmsManager and hook up some verification code on "UpdatePage" command. For example, you may compare the title of the page (let's say it's the name of the store) with the email or ID of the user that has made the modification. Then verify that against some lookup table and send yourself an email in case the person making the changes is not a store manager for that store. Also, there is a static "Executing" event, which passes you Cancel argument, so you can cancel the update and redirect the user to some page telling him/her that pages of other stores must not be edited.
I am sorry that Sitefinity could not exactly fit your given scenario, but I hope you'll find the workaround satisfying. Let me know if you have some additional questions.
All the best,
the Telerik team