[Rhod the Red]

The social system of Elvish society is not covered really, an exception to the other races in his books. Other than absolute monarchs(not regressive), like in Mirkwood, Elves appear self-governing. Rivendell appears to be without government as we know it. The fact that some Elves are called 'lord' and 'lady' may just be a reward for brave deeds or something.

Calling Boromir a 'lord' of Men would be correct in the context Tolkein usues it, other trhan rulership. It can mean a great warrior or wise bystander, in Tolkiens usage