The Great Man Theory or Woman as the case may be, is not so much that they were a strong personality, though that is a apart of them, but that they could see possibilities when most could not get beyond the status quo. Napoleon realized during the Royalist counter-revolution, that if he sided with the Royals, he might become a sergeant, if he sided with People, there was no end of possibilities.
How many people lead dead end lives for lack of vision.
In the case of Justinian, you left out the great woman at his side, Theodora, who without he would have surrendered the throne and fled. That experience of a woman, his wife, willing to die to remain an empress, gave him the backbone he needed.
He also had the willingness to look to talent over those well placed by birth, money, or connections.
A rare ability indeed.
Yes the times may mold the situation, but a bold but brilliant person may take charge and change it beyond all recognition.
It is beyond our experience as a species to have any record of a horse, dog, money, or cat, so changing the world of dogs, monkeys, cats, or horses, that they behave differently. Start doing different things, eating different, perhaps an expanded diet. Yet within the realm of history, over the past 5,000
man, and woman have done the same thing time and again. Whether Julius Caesar, Napoleon, Jesus, Mohammed, or Mary, Queen of Scots.
She having become a queen at about six weeks old, maybe less, was able to make things happen that she wanted to. On the day of her wedding, instead of the normal red wedding dress, to symbolize the combining of the bloods, she choose white. Not liking how she looked in red. A little thing, but it changed the bridal gown from red to white, a tradition that went back to Rome and perhaps the Greeks as well. Few are those who in a day, can overthrow a thousand year tradition, and make it stick.