Hi, welcome:
As to question 1 -- I don't know of a way around the limitations of filtering in browser forms. I'm sorry. Here is an article on the InfoPath Team Blog for writing a web service to return filtered results. If you don't want to write a web service, we sell a very complete one here. If you aren't a programmer and this kind of filtering is essential, I think you'd find it to be of great help.
As far as your proposed design for the workflow, I often have fields in my main data structure that I don't put on the form. You can use a rule on Field 1, so that when a user selects a Bureau, the Backup field is populated....Perhaps you have that information in the same data source as your list of Bureaus?