Your first sentence might be clearer if broken up. In addition, I think that you might mean that they had no choice but to endure the harsh conditions. So, I would say “…Islanders had no choice but to endure the harsh outback environments. They had no choice because they had signed three-year work contracts even though…”
“Those who had not died of homesickness” doesn’t quite work. You can change it in one of two ways. First, you could put a date in; “Those who had not died of homesickness within the first X years…” Second, you could simply say “Those who did not die of homesickness…” Also, remove the word “they” from this sentence.
Decide whether you are going to capitalize “kanakas” and be consistent with that throughout the essay.
Finally, I still think that you need to justify your use of the words “enslaved” and “slave labour.” You have not made it clear that the islanders were actually slaves.