The ʻana does turn the "verb" hui (get together, meet) into a "noun".
A better translation for "the farming of the land" might be "ka mahi ʻana i ka ʻāina" rather than "ka mahi ʻana o ka ʻāina", because using "o" gives the impression that the land itself is farming something, rather than something is farming the land.
Both mahi ʻana and mahi ʻai ʻana are possible. As a noun, mahi is a farm, plantation, and mahi ʻai refers to a farmer, planter, as well as the action of farming. And ʻai means "food/edible/to eat", &c, and figuratively, "to rule, reign, or enjoy the privileges and exercise the responsabilities of rule, and one who does so". Ex.:
Hale kūʻai= grocery store;
ʻAi āina=to own, control, and enjoy land; the owner of land.
im not to sure on if these lessons are for writing Hawaiian properly or if its for speaking it but I know from growing up in a Hawaii that speaking and writing are pretty different because before Christian missionaries came to the islands there was no written language so speaking can be very contextual not needing so many words because your body language and the situation your in fill in the gaps compared to writing where you need to know more about whats going on to understand whats being said
There are a lot of languages that rely heavily on context be it written or spoken. The most important part about learning a language is learning to identify context clues and reading between the lines. You dont get to do that if you dont even know the words to begin with. Straightforward languages that dont carry any context in their words are boring anyway.