Magic Moon wrote,
As for the Venetian lion on the King of Swords' shield, I think it reflects that Francesco had been an important military figure in Venice. That card could have been painted that way even if the deck wasn't created for someone from Venice, but I understand it may strengthen Venice's argument. I don't know what to think anymore. Thanks for considering the suggestion.
Another argument against the deck's being made for William VIII of Montferrat comes from looking at his biography on Wikipedia. It says:
In 1449, William was accused of treachery and imprisoned by Francesco Sforza. The reason was said to because he had formed an attachment to Sforza's wife Bianca Maria Visconti.[2] He was released a year later. Angry over his treatment by Sforza, William encouraged his brother John IV of Montferrat and relative Louis of Savoy to make war on Sforza (and Milan), and then he himself would lead the troops in battle. This attempt failed.
It adds:
William later became a tutor to the couple's son Galeazzo Maria. Following his assassination, William held several positions in the Duchy of Milan.
About Elisabetta Maria, Italian Wikipedia says:
Fu destinata a sposare, dal fratello Galeazzo, Guglielmo VIII Paleologo, Marchese di Monferrato (1420-1484), già vedovo di Maria di Foix.
He was destined to be married by her brother Galeazzo, to Guglielmo VIII Paleologo, Marchese di Monferrato (1420-1484), already widower of Maria di Foix.
This "destiny" was then opposed by their mother Bianca Maria. It was surely due to Galeazzo Maria that Elisabetta first became engaged and then married to William, after the death of their father.
I had thought that perhaps the six added cards were commissioned by Galeazzo Maria to honor his sister, and out of guilt for insisting on the marriage - not because the groom was too old, but because his sister was too young and small (or frail) to safely bear children at that age. She died from complications after giving birth. If, as you say, the lion was to honor Francesco Sforza for his former service to Venice, that would still be possible, the deck being given to William, for which a red and gold coat of arms would make sense.
The putative artist, Russi, was probably by 1472 at Urbino, but he still could have done it - or someone else, as it does not seem to me that there is anything that uniquely identifies him as the artist. If it could be shown that Russi also did the "Mantegna" cards (quite possible, since he was in Ferrara then), there would be more of a case for his doing the added cards, since there are similarities in composition. But that does not much affect the probability of their being done to honor Elisabetta.
However, the Ace of Coins is one of the first-artist cards. Does it make sense, given Francesco's animosity to William, for him to have been given a deck of cards with his coat of arms on it by the time it is estimated to have been done? If William was tutor to Galeazzo, he might have gotten back into Francesco's good graces, if that job started while Francesco was still alive. Galeazzo Maria was born in 1454, so it is possible. Would William have merited a deck by 1456-58? I suppose it depends on when peace was secured between the two realms, and that I don't know. The Bon hypothesis seems more likely, given that Russi was established in Venice and the prevailing view that the colors were red and silver.