Which countries have the highest population on each continent? Where countries straddle two continents, they could appear in either continent's list - but only the population within each continent is counted.
Mainland and offshore are relative terms, aren't they? When landmasses are next to each other, people tend to describe the bigger one as the mainland and the smaller one as an offshore island. Which means that Barbados for instance is generally considered offshore of North America, but is in turn the mainland next to the one other tiny island of that country. Here mainland means the world's two largest landmasses, and everything else is offshore. Oceania should be a cinch without having to think about that, cos you only have one box to fill.