A lawsuit will take years, and Foster provides quite a good reason to suggest that delayed recovery has substantially discounted value, so while that would be a possible route to recovery (probably for his estate by the time it was resolved), a quick resolution now, even if involving less money than a successful suit would recover, would probably be strongly preferred.
That said, I would think that filing suit would be the way to kick that process off, as that would reduce the perception that "ignore him and the issue will go away" might work.
Wouldn't be the first time when delaying tactics were used by big corp to make it so that there would be zero payout through some BS or that the suing person died.
Whomever owns the copyright is entitled to do whatever their control of it allows them to do, but they are obligated to fulfill any claims others have on the revenue derived from those uses.
In this case, the book as a whole would be copyright Star Wars Corporation, probably a holding entity specially to handle multiple involved copyrights (the copyright to Star Wars itself, etc.), and unlike a Windows install media you won't find a page listing "this work contains parts copyright X, Y, Z". But the contractual obligations still exist.
A lot of "work for hire" contracts don't pay royalties, just a flat fee, but I don't think we'd be talking about this at all if this were the case here -- Foster was evidently being paid royalties until Disney took over Lucasfilm. So the dispute is over the second question. Disney is implicitly arguing that they get out of paying royalties to every single author who created works for companies that Disney subsequently bought, regardless of whether those contracts specified royalties that the predecessor corporations would have otherwise been paying.
On top of that, these kind of contracts often come with a 'flagrant breach of contract means that copyright reverts to the original author' type of clause.