Hi HappyDigit and welcome to our forum!
I think you are on the bleeding edge here. We have an Active Directory Web service that we sell to people and is bundled in our Database Accelerator suite. That allows you to query permissions. The Database Accelerator Web services can set user roles and permissions based on Active Directory info. By using UDC files (universal data connections) and Office's new SSO authentication, it's possible to create data connections for these Web services and then include them in the InfoPath form you intend to display in the browser using Form Server. In this case, the roles will be set based on values in your form, or in meta data that is stored referring to your form. After setting this all up, in theory you should be able to add open rules that switch the view based on whether the form was opened read-only or not. You can have rules that also check if the user is in a certain group and change the view based on that information.
I will say that it's a lot easier to do this with the rich client. Form Server does not support everything (as you have found out) and one of the key value propositions of InfoPath is the ability to offload server processing, support offline access, and provide a richer editing experience. User Roles require domain authentication and that is more complicated with Form Server since it works as proxy between the browser and the data connection.
Patrick Halstead
Project Manager at Qdabra