I have no doubts now about the Grail connection of the suit of Cups.
Pictorial Key to the Tarot concerning the Six: "A card of the past and of memories, looking back as--for example--on childhood; happiness, enjoyment, but coming rather from the past;
things that have vanished." (As usual in
PKT, Waite leaves the best for last.) As explained in Mary K. Greer's
The Grail and the Minor Arcana--the Evidence, and elsewhere, the Minors tell the story of loss: the disappearance of the Grail and the loss of the Word (due to the death of Hiram Abiff) in Freemasonry. Cup, Spear (Wand), Sword, and Dish (Pentacle) are considered by Waite to be the Grail Hallows.
A card of memories, the Six depicts a happier time when the Grail was still among us. However, the origin of the Cup is not to be forgotten. The processional figure with the lance probably was intended by Waite to refer to the Spear of Longinus, which caused the wound from which flowed the (metaphorical) blood and water of redemption, which (according to legend) was caught by Joseph of Arimathea in the Sacred Chalice.
ETA. The blood shed by the Christ is considered by Christians to be the supreme gift of God to humanity, our own unselfish gift-giving (such as the cup of flowers offered by one child to the other) being a reflection of that ultimate gift. While the Grail in Wolfram's
Parzival is not related to the Holy Vessel, Robert de Boron linked the Grail to the Christian myth in his
Joseph d'Arimathie, and there apparently is evidence that Robert was one of Waite's sources.