So this is another alt-historical way in which the Empire could have totally screwed human history in interesting directions that came to me in a dream last night, and the best part is that it only requires them to play Alien Space Bats for a while. The agency in the really fun parts belongs to the humans.
So, point of divergence-wise, what we have is an island in the Atlantic, and a damaged starship that needs to set down for repairs. (Really extensive build-the-things-to-build-the-things repairs that might keep them there for a century and some change, even if that means having to rebuild stuff from scratch. Apply fudgery as needed.) But really, we don’t need to detail this too much because they’re not trying to have any extensive interaction with the locals, just to fix stuff and go home, and can apply keep-aways as needed to get some privacy on said island.
Unfortunately, the time they’re there is around the 1700s-1800s, which means there’s all these slavers sailing right by their south coast, and they like that about as much as you would expect.
It does not take long at all for word to get around that trying to take a slave ship across the Atlantic is basically inviting the fucking gheallie sidhe to come riding out of the sky complete with storm, fire, and 37 flavors of bloody vengeance on their minds.
It doesn’t take much longer for large parts of the Carribean and the southern American colonies to start finding a different business model once plantations start burning to the ground, entire workforces vanish, and there’s not a goddamn thing you can do about it since bullets don’t hurt the fae, belike. Not even thrice-blessed cold iron ones, and that’s a hell of thing. Whose side is God supposed to be on, anyway?
Meanwhile, since they have to do something with everyone they’ve liberated, and they’re often not in great condition, there’s an entirely different kind of “Liberia” coming into being on the former slave coast of West Africa, and while they aren’t in any position to pass along the entire technological toolbox even if that seemed like a good idea, there are plenty of old ideas, easy to make at home, they could pass along that would greatly improve the quality of life thereabouts.
It is sometime in the 1800s when they return homewards. In the meantime, the Atlantic slave trade is entirely dead, and even if the sidhe have left no-one wants to invite them back. Social evolution across half the New World has taken an abrupt swerve sideways in the last several decades. And West Africa is now dominated by a new nation that’s picked up at least some of the attitude, has steam power, railways, telegraphy, antibiotics, and better guns than just about anyone else on the planet, and is not interested in hearing your ideas about colonization.
Go.