Changes in DNN do not get written back to the AD. It's a one-way trust (information is read from the AD only). As far as updating email in the AD and it not reflecting into DNN, were you already logged into DNN when you made the change? It's not an instaneous update and can take up to an hour before you see the change. To double check you can try clearing your cookies and cache and then going to the DNN site to see if the update has been made to the DNN account.

The AD provider is basically meant as a means for your AD users to be able to login into the DNN portal without having to create a separate account like they would if there wasn't an AD provider. After that the DNN portal works just as a stock DNN portal does. So, if you make a change in the Edit User screen it only applies to the DNN portal. If the change happens to be one that is synchronized with the AD when the user logs in the next time that change will be overwritten by what's in the AD.

As for your AD user without an email ID. Unfortunately you need to have an email address for DNN. It's one of the required fields and when I last checked you couldn't make it optional (it's been awhile I'll admit). In the past there were problems caused if an account didn't have an email address (such as viewing the Users page). An error would be thrown on the website. So, if the AD user doesn't have an email a proxy email is put in their place. In other words, you can't stop it from being populated without changing the provider code and if you change it could lead to other problems on your portal.