You're quite right, technological advances don't necessarily correlate with changes in values, and "progress" is a complicated concept.
Technological advances do occur in unequal societies with limited individual freedom. Otherwise we, in the real world, wouldn't have gotten to this point, where we can have this conversation. But, I think such progress would be a lot easier in a more equal and free society.
Centauri society is apparently quite patriarchal. We never see any Centauri women in government. To be fair, Centauri patriarchy could be a system where scientist is an acceptable job for women, but it probably isn't. Especially not seeing how Londo's nephew was put in the position of lead scientist at Ragesh III. In a system like that, 50-ish percent of your society's brain power is automatically excluded from working on things. Then, among the 50 percent of people who could potentially be working on things, you have people legally killing each other in duels. All of this against the background of scheming that is Centauri politics. It's amazing they ever get anything done at all!
You could explain it by saying that the Centauri have taken a VERY long time to get where they are (Which may or may not be a known thing. *I* don't know), or that they have indeed gone back to older ways of doing things. I know that the humans got jumpgate technology from the Centauri, but I don't know if they came up with it themselves or got it from someone else also. That could explain things, as well. Maybe the scenario was more like the Narn, which, as you say, had technology thrust upon them they weren't anywhere near close to developing themselves.
As for Centauri imperialism, looking at human history, certainly imperialistic nations did quite well for themselves at different points in time. But, they did so in an environment where there wasn't any powerful opposition to that kind of exploitation. The Centauri are being imperialist in the context of an interstellar society that frowns on that sort of thing. There's all kinds of treaties and things ("Ink on a page!"). Seems like it would be more trouble than it's worth to maintain your imperialistic tendencies in that kind of environment. At least not until you partnered with the Shadows