Followup: I continued teaching my daughter fractions and decimals which had two consequences: she is now in the top maths group which is taught by a different teacher, but now she is the dumb kid in a group a smart kids.
The stunning scenery on Shengshan Island is the result of the houses and outbuildings being slowly consumed by nature. The seaside village now lies empty because it was more economical for the fishermen to move and work on the mainland where catch could easily be transported and sold.
The first option is to have monsters responsible for the Greenlanders disappearances. The second option has Portuguese slave traders, working in conjunction with a colonist, responsible for the disappearances of the Greenlanders. The final option has the Greenlanders evacuating the colony peacefully and traveling to a new home in Vineland.