Actually, in my expert opinion, Lady FallingStar did not technically err in choosing her form of address.
Of course, it is within His Excellency's purview to choose whatever form he prefers. However, if one consults the literature, one finds that use of the noble's last name is in fact rarely used in conjunction with such a form, except when the noble title is also included and delineated with a comma.
For example, usage of the following forms has been noted in official records:
Baron Darien Fairfax;
Your Excellency;
My Lord Baron;
And occasionally, for particularly formal documents:
Your Excellency, Baron (Darien) Fairfax.
Other forms of address are sometimes employed, of course, but these are the most common. Isn't that interesting?
--Evangeline
Chief Librarian of the Fairfax Historical Library