Using windows authentication followed by forms authentication with WCF & Custom membership provider

My server is exposed to the clients via WCF.
I have 3 types of clients 1)ASP.NET 2)WPF 3) Powershell scripts
My task is to provide authentication mechanism that will use Windows Authentication on first and only if it fails , the client will raise a form asking the user to enter user/password.
The same authentication mechanism should apply from clients to the WCF service & from the WCF service to the Active Directory
I will also like to use my Custom Membership Provider that is inherited directly from Microsoft ActiveDirectoryMembershipProvider.
I need an advice for the best way to accomplish this task.
Questions :
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1) Can I trap a Windows Authentication failure in order to raise a login form/window.
2) Is there a way to configure WCF for both Windows/Username options without two separate endpoints.
3) Does it make sense to use WCF settings + Custom membership provider for this task.
4) Can I use a client with no provider/Active Directory settings where the client authenticate itself to the WCF service and only this service verifies the user against the Active Directory

Open Issues
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1.      I have didn't found any way to support in powershell command scripts both:
a.      Login(user, password) command
b.      Login() command (using Windows Authentication)
Only one can be supported for a given client.
2.      I have found no way to use only one end-point and still enabling clients to connect with both Forms/Windows methods.
3.      Seems that, when using Windows Authentication, the service connects to the Active Directory using the credentials of the user that runs the service. That means that we will have to define the NETWORK SERVICE under which the service is running as an Active Directory user with read privileges.

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