I see what you're saying about her maybe not having been to Earth at all, I think that is a distinct possibility. My primary evidence for thinking the opposite is the tone of voice she uses to say that she never believed in the Earth, which is frustrated, and almost dejected, like she had the chance to believe in it but made up her mind a long time ago that it wasn't worth it. This could be an indicator that she had heard of it while off planet, or, as you mentioned, she could have seen it through the mirror (which is actually a really cool idea, if you are right, I hope they do an episode on that!) but I'm still slightly more inclined to believe that she had a personal experience as a pre-mirror gem.

From what I can recall, there seem to be two more examples that might be similar to Lapis. The desert glass (which they embedded in a pillow?) and the gem in the temple which seemed almost grateful to be taken out by Steven. I wouldn't be surprised if they found a way to explain what happens when gems are embedded in something, since Steven himself seems to be a consensual and sentient form of this behavior.

If I had to guess, I'd say it's something similar to slavery? I have a hard time believing that it is a black market of sorts (given that Pearl seemed perfectly calm to talk about Lapis' mirror and we all know how she reacts to rule breaking). When Pearl describes the desert glass, she talks about how it is building in chaotic ways. All the monsters are chaotic, but this one seems more like a broken tool than a sentient being who became beastly.

But yeah, the point you make about the dissonance between imprisoning gems and Lapis wanting to return home is an odd one, though if I may, it's one that makes me even more inclined to believe that whatever happened to her, happened on Earth (or potentially some other planet, but Earth seems most likely). After all, if you were grievously harmed by someone, wouldn't you want to escape as soon as possible? Maybe in a less than logical way?