I have only just started Bunpro and have about 30 study items started. When I do reviews however, there are questions for items which I’m sure I didn’t cover. Could it be because I imported my WaniKani progress?
Eg - 
Also, this review question provided the answer right there too. What’s the purpose of this?
If you scroll down after answering it will show you the relevant grammar point (actually maybe you have to turn that on in the options - I’m not sure) but I am 98% sure that the 大変 example you posted is for the “な-adjective” lesson as there is no other context in which it would make sense. In fact, if you just started then I am 99.9% sure. The purpose of it is to know that “な-adjectives” act like nouns in sentences like the one in the example.
I can’t speak to the getting questions for things which you didn’t cover but try checking the grammar point that comes under it next time to double-check if that is actually happening or not.
Edit: Do you have any examples of getting questions for things you think you haven’t covered that you could post?
You’re right, it’s in the な-Adjective examples. I had just assumed the examples section was additional info, like how it’s used in WK, and not something to study as well! Thank you.
Do you have any insight as to why I’m shown the answer in the question in that example?
If I understand you correctly, the word in brackets is there so that you know which verb/noun/adjective you have to use in a given exercise. In this case you just had to type in exactly the same word, which seems strange, but actually isn’t.
Yes, here it’s the same, but for different types of grammar points, you may need to add in a different ending (e.g. -katta for past tense)