You write: "I'm also not sure why we [...] have to hear about the art returns"
The explanation for mentioning the history behind the art returns is clear. They mention that they believe "that Marvel retains in its possession certain Kirby Artwork that it did not return to Kirby." They allege that "Marvel concealed and continues to conceal that Marvel retained certain Kirby Artwork that it did not return to Kirby". Therefore, they conclude that Marvel's actions "justify the awarding of exemplary and punitive damages."
My impression is the Kirby family is holding Marvel responsible for all of the Kirby art done for Marvel that was "lost" or stolen by staffers back in the day (as well as artwork that may still be in Marvel's possession).
They're also asking for their share of profits from "derivative works" such as the recent Hulk and Wolverine movies (movies according to them derived from concepts created or co-created by Jack Kirby).