Could be interesting but since so much of this is up to the judge, and laws that covered these things back when Disfarmer passed away. There was no automatic copyright until the 70s. It has been 62 years. Any rights that the 60 relatives have to these photos and negatives, will go Public Domain. (unless they already have for other variations in the law?) Wait a minute! 60 relatives pop up for someone who appears to have been kind of a peaceful, solitary person.
Ambulance chaser, finds relatives, to sue... of course he does get a percentage. Then he makes what appears to have been a reasonable deal and the family (loose term for distant gold diggers) fires him.
Not a legal point, but where were all these unknown relatives when Disfarmer passed away. I'd bet that most of them, didn't know they were relatives until the attorney started hunting for heirs so he could file.
Bottom line, if these were a business and that business dissolved, the copyright protection ended. If something more likely, and these are works of an individual, then: Life of the author + 70 years.
The heirs had a chance to take an offer, and declined. I'm wondering in the long run, if this ends up in the courts for say eight years, if they just lost everything and will get nothing. Complications could be infighting, with some people claiming the others aren't entitled to part of a settlement. Appeals, and that expense and again the worst enemy of their claims is time.
I really don't expect the fad of popularity to last, nor will the high values for discovery and marketing. At that point, which may already be here, the art museums and people who appreciate the work for it's content, instead of investment, won't be willing to pay a huge sum.
Where laws are unclear and there isn't a higher court decision, or a number of them, agreeing, the claims will be decided by a judge. If that person is sympathetic to the claims, then the family could win. If not, they will have spent time and money and get nothing. Settlements are usually made to save the expense of a defense. The family turned down the settlement. No, if it gets to court, in under eight years, it's going to be up to a judge. Then depending on how big of a fight the losing side wants to put up, a higher court, and a higher court...
I'd love to see tougher laws and better protection. Laws have changed a number of times recently, yet DMCA and the US laws have no teeth and don't protect us as artists. The reason music and video/movies have such nice protection now, in the digital age, is that big corporations had an interest in protecting their rights and future profits. There doesn't seem to be a big interest in protecting artists, designers and photographers. We aren't a big corporation that can lobby and apply pressure for better laws.
Go try and file an infringement claim when you see that someone has stolen your work and is using it, without the rights or with the wrong kind of license. Good Luck!