I need to be careful because my knowlege is jurisdiction specific.
But assuming there are strong similarities between my jurisdiction, and the jurisdiction whose laws apply to Vivian Maier's estate, the issue of copyright is inextricably intertwined with the issues of probate. This is because the copyrights are the assets of yhe estate, and the probate court has jurisdiction over those assets.
If Mr. Maloof corectly identified the person or persons who are entitled to Ms. Maier's estate, then he may very well have settled the matter with the people who may eventually have authority to settle it. Except in my jurisdiction, when someone dies without a Will, as Ms. Maier apparently did, then no-one has that authority until the probate court grants that authority.
I have seen nothing that indicates that that has happened.
Even if it has happened, there remains in the court jurisdiction to later correct an error, if information should become available that indicates either that the person given authority (the administrator) ought to not have been appointed, or that any distribution of assets (such as copyrights) was unfair or incorrect.
There are a whole bunch of rules about what can be done if an error is discovered, but suffice it to say that the law only requires correction of what can fairly be required.
If Mr. Maloof found what he believed to be a single, entitled heir but did not protect himself by first having a probate court pronounce on the validity of that determination, he was unwise. Because he may have entered into an agreement with someone who has no legal authority to do so.
If there was a determination, and it turns out to be wrong, then the other rightful heir(s) surely have the same rights to participate as the distant relative that Mr. Maloof located.
The art world values authentication and provenance. A court application may be able to provide certainty where currently none exists.
By the way, the person to "blame" for this is none other than Vivian Maier. If she had executed a valid Will, none of these issues would have arrised.