Where is the eaten verb in this sentence?
"Déjeuner" is a verb meaning "to eat lunch" or "to have lunch."
And also a noun? Until now, that's the only usage I've seen
Yes, "un déjeuner" is a noun with its article, as well as "un dîner", which can also show as the verb "dîner".
I'm finding this entire lesson very difficult and frustrating. It would really help to have some hints and tips on it to actually teach us what we're supposed to understand.
The construction venir + de + infinitive is used to say that an action has just happened. It's called the "near past" or "recent past."