Clientless mode failing to interact with AD

As you remove the session you kill it. You should not wait for access completed to do something. Either you put a 401 challenge or you use an irule event to respond with 401.

Having said that talking about webservice you may expect login and password to be already in the request. In that case you create an irule event and in the irule you retrieve login and password to assign it to session logon variables.