In conditional tense both ville and skulle (and other auxiliaries) can be used, this sentence is in 2 conditional tense (skulle/ville + ha + participle of the verb). However, ville suggest there's a condition missing, and the sentence is incomplete without it. Skulle works good on it's own here.
De ville ha spist lunsj i dag, om ikke bussen var forsinket. They would have eaten lunch today, if the bus hadn't been delayed.
Kommer att is Swedish (de kommer att äta lunch), in Norwegian it would be kommer til å. But De kommer til å spise lunsj = they will eat lunch. To get the right tense = De kommer til å ha spist lunsj. It's a bit strange to use kommer til å in this construction unless it is an answer to a question, de vil ha spist lunsj stands better on it's own.