In that case the rebels would only have the time between when the school was attacked and when the blockade went into effect to set up a self-sustaining operation. Not a lot of time for anything more interesting than moonshine distribution (on a scale of illegality).
After some thought, I think the lack of rebels on Greenwood might be justified because as the last isle in the chain, it has no strategic value to the rebels after the school is destroyed. You can see this when what you do with the Greenwood Creator doesn't change your Shaper/rebel standing, but what you do with the Dhonal Creator is a major path choice. Again, having the rebels around to interact with sounds fun, but I'm not sure they have an in-game reason to be there.
Player choice in this case ties into the poor writing. Lankan comes across as an idiot while the game is trying to present him as a leader because the root of the conflict between him and Diwaniya, as written, is completely solvable with Diwaniya's solution (together with Norrell's offer to negotiate a lighter sentence for Lankan), and the writing doesn't acknowledge this. (I did point out that the game's ending shows that removing the rogues doesn't get Lankan to submit to Diwaniya, which is consistent with how their conflict was written, but has no bearing on how illogical the conflict is.) This is still true of your solution. I would ask your Lankan the same question: Why should I support the outpost by breaking Shaper law and bringing them research and recruiting for them (which takes much more time, as well), when I can remove the rogues or build up San Ru as Diwaniya wanted and make the island safe for everyone?
Also, you pointed out yourself when talking about Barzahl in G2 that he would be more interesting if he was pursuing power for the sake of what he thinks is progress. Therefore, it is entirely possible for Lankan to have sympathetic reasons for wanting a canister as a means to his ends, if the conflict was better written.
An interesting point here. Because there aren't any Shapers other than you, the NPCs rightly or wrongly place the blame on you or expect you to fix things. One problem this creates is that Greenwood's narrative of "Shapers aren't responsible and don't care about outsiders" clashes directly with the typical RPG player's overly helpful (if not entirely altruistic) disposition when a game has free saves and no time limit. The best example is the dialogue where the commander is trying to twist your arm to deal with the Creator. You have dialogue options to be utterly indignant that she would dare to tell a Shaper what to do, but at the time, I was wondering: "Why would I say that? Killing rogues is in my job description, and even if it wasn't, I killed worms for like twenty people on my way here, I can make time (and get XP) for something that is actually causing you problems!" If a typical Shaper in fact gets indignant at this sort of thing (and the commander in fact usually needs to manipulate them to get things done), there need to be typical Shapers to show this.