The thing is, the Romans would have no idea of how they worked, or their limitations. One huge demonstration outside the walls of Rome, destroying some abandoned structures with artillery, slaughtering thousands of sheep, cattle and pigs from the air, and you could bluff them into damn near anything.
I don't think a single human would need to be killed to effect major changes in their society.
Then there is always the God route, openly accept that you were chosen by a single great entity to rise Rome to rule the world but they would have to change their ways first. Admit you're just humans like them, but you were chosen and hand picked because God was upset his son was killed and he was going to give humans one last chance to straiten up or he would destroy the world and start over.
You could install western values in a couple of generations and with the technological advances, hell just accurate maps, you could bring the whole world into a age of enlightenment.
Basically, you become the UN but rule it with an iron fist.
A MEU has enough supplies to sustain itself for a week. In a situation like this they could stretch it out to a month or two on the outside a year. Fuel and ammo are finite.
If I were the MEU CO I'd get on the ships and sail back to America go to Titusville, Pa and drill for oil. With oil all things are possible. Within a few years you're self-sustaining and then go back to Rome.....
The colonel commanding the MEU would have to combine cunning with a level of ruthlessness not seen in Western warfare since the SS were defeated. He would also have to be able to manipulate Roman religion, supersticions, etc. You are after all an alien society that has a greater understanding of your target than they have of you. That is your chief advantage.