I think it's important to bear in mind that we're really looking at a number of distinctly separate copyright entities, i.e.:
1) Pamela Colman's Smith's original artwork (or direct copies thereof)
2) The deck of cards published by William Rider & Son of London featuring coloured versions of Colman-Smith's artwork [1909-1910]
3) The Key to the Tarot, i.e. text by Waite [1911]
4) The Pictorial Key to the Tarot, i.e. text by Waite along with Colman-Smith's artwork in monochrome [1911-1912]
The easy stuff first: Waite's text for The Key to the Tarot is undoubtedly in the public domain in the UK/EU, and the same is almost certainly true of its adaptation for The Pictorial Key.
Although technically a co-authored work by virtue of text by Waite and art by Colman-Smith, the two elements can be clearly divided, since I doubt there is any suggestion that Colman-Smith had any hand in the text by that stage.
If we put aside the issue of the underlying artwork, there can be no lasting copyright in the cards as produced by William Rider & Son of London, unless an individual can be identified who was principally responsible for their presentation - i.e. the colouring, design implementation, etc. - and that they died later than 1944. If not, as an "anonymous" work, copyright will have expired 50 years after publication, i.e. after 31 December 1960.
Everything, I think, hinges on exactly what the arrangement was between waite and Colman-Smith. Plenty of sources simply describe a "commission," but it's clear that Waite had more than a little input in what was on the cards, and others may be able to clarify just how collaborative the process was.
As ever the law hangs on strict definitions, and there are clear differences between how copyright ownership (but not term) is treated in light of whether it was produced in the furtherence of someone's employment, as what we would term a freelance commission, as a voluntary service, or if the copyright was assigned to someone else after the fact. It seems to me that Waite's motivations were more to do with getting an "ideal" Tarot deck into circulation, rather than as a money-making exercise, and that Colman-Smith's involvement may well have been on the same basis. Obviously if there is in existance an actual legally-binding agreement confirming a commission, with Colman-Smith forgoing any and all claims to her artwork, then Waite's successors would still be the copyright owner. If, on the other hand, there is documentation confirming Colman-Smith merely assigning any copyright claim of hers to Waite after the work was done, ownership would have reverted to her successors 42 years after publication, as that was the copyright term pre-1911 Act.
I've had a chance to look at my copy of Tim Padfield's Copyright for Archivist and Records Managers (a standard work on the subject), and it does confirm my previous understanding of the 1911 Act on this particular point, and that it was preserved in both the 1956 and 1988 Acts, i.e.:
"Substituted rights under the 1911 Act
The 1911 Copyright Act changed the duration of copyright for some existing works. If copyright had been assigned for the whole of its term by the author before 1 July 1912, the Act said that the assignment was to have effect only for the period of copyright that applied at the time of the assignment. The right for any extended period created by the new Act were given to the author, with only limited rights remaining to the assignee, unless the author made a new assignment after the passing of the Act."
This clarifies a point I was previously unsure about, i.e. whether a previous assignment could still stand, if properly worded (e.g. giving rights "in perpetuity"). Instead, it seems that if Colman-Smith had assigned her copyright to Waite in 1909, they would have had to make a new and separate agreement after 1 July 1912 in order for the rights not to revert to her at the end of 1952. I suspect few people would think that likely to have happened.
In the absence of any sort of agreement one way or the other, it would really be down to a court to decide the exact nature of the transaction between Waite and Colman-Smith. It could be argued that it was a de facto commission, and so should be protected as such, but equally a judgement could err on the side that in the absence of commission documentation it could only be seen as an assignment, and thus no longer owned by Waite's successors.
Obviously IANAL, but my own opinion would be that if no documentation confirming a legally-binding commission exists - or only documentation confirming a subsequently reverted assignment exists - anyone claiming ownership of Colman-Smith's copyright (other than her own successors) would be ill-advised taking the question of the ownership to courrt. Even if they possessed documentation confirming a legally-binding commission but did not disclose it beforehand, the court would probably take a dim view of them waiting to use it at that later stage. A trzes says, we really need to know more about the US Games' supposed victories in court, as they may clarify what original documentation they used to back up their claim.
ETA: The pre-1912 Act term was 42 years from publication, or seven years after author's death, so in Colman-Smith's case it would be to the end of 1958.