At least Romulus was undoubted a Roman, he is the dude who founded the city of Rome.
The end of the Roman Empire is not so easy to apoint a date or a leader of the last emperor of the Roman Empire.
Maybe the West part of the empire ended with Romulus Augustulus, as you said, but it was revived by Charlemagne as the Holly Roman Emperor.
And this revived West Roman Empire, called him self the Holly Roman Empire, just fall down in Napoleonic wars when the Holly Roman Emperor Francis II decided to end it's empire because he lost the war against Napoleon.
The other half of Roman Empire maybe fall with Constantine XI, as you said, but the Turkish proclaimed they self Roman emperor by the right of conquest. Let's remember at the time the Austrian proclaimed it self as the Holly Roman Emperors too.
And it also has the third Roman is Moscow
, who proclaimed it self Romans as the right of religion (they follow the faith of Constantinople).
And for last, the Mussoline also proclaimed it self the Roman Emperor. (he was an emperor in Rome). So that can be understood the Roman Empire just fell in 1945 when Mussoline lost the second world war.
And since the Roman Empire still in our imagination, it can rise again someday in the future (I just hope it's not happens agains).
About the animation budget, Fireaxis can make the animations less hard to do, something more cartoonish should be able to do more leader per civ.
Fireaxis already try to do something similar in Civilization 3, where the leaders trade his clothes per era. But I don't like this way to do it, I think is better changing leaders (in order to they change their behavior when be controled by AI).