The Leber Star card doesn't just show Venus. There is a prominent ship in the background. MJ Hurst in 2009 analyzed that association correctly (http://pre-gebelin.blogspot.com/2009/04 ... arot.html; but be sure to read what he says in the "Comments" section), citing Frazer's Golden Bough:
The attributes of a marine deity may have been bestowed on Isis by the sea-faring Greeks of Alexandria. They are quite foreign to her original character and to the habits of the Egyptians, who had no love of the sea. On this hypothesis Sirius, the bright star of Isis, which on July mornings rises from the glassy waves of the eastern Mediterranean, a harbinger of halcyon weather to mariners, was the true Stella Maris, “the Star of the Sea.”
It is not the Egyptian Isis, unless you count the Greeks in Alexandria, who lived there for centuries, as Egyptian. It is the Greco-Roman Isis of the Roman Empire. Most educated persons in the Renaissance would have known about her role related to ships from Apuleius's Golden Ass, whose famous Book XI describes a procession to the sea by the devotees of Isis; I highlight the most important parts (http://www.poetryintranslation.com/PITB ... nAssXI.htm):
Meanwhile amidst the tumult of the festive celebrations we had slowly progressed towards the seashore, and arrived at the very place where as an ass I had been stabled the previous day. There, once the emblems of the gods had been properly disposed, the high-priest consecrated a finely-crafted ship decorated with marvellous Egyptian hieroglyphics. Taking a lighted torch, an egg, and some sulphur, he uttered solemn prayers with reverent lips, and purified the ship thoroughly, dedicating it, and naming it for the Goddess. The shining sail of this happy vessel bore an inscription, its letters woven in gold, the text of a prayer for prosperous sailing throughout the new season. The mast of smooth pine was raised now, tall and splendid, the flag at its tip conspicuous from afar; gold-leaf glittered from the stern which was shaped like Isis’ sacred goose; while the whole hull of highly-polished citron-wood gleamed pale.
And it sails out of sight on convenient breezes. I assume it is a small model, as the people put offerings on it and it sails of its own accord. Of course Isis's statue was not naked. The card is an amalgam of Venus rising from the sea and Isis as goddess of sailors. Amalgams were popular then. A good discussion is of two tomb reliefs in Florence that combine Helios and Eros, and Mithras and Hercules, according to Frederick Hartt, the art historian who wrote about them (see my post at viewtopic.php?f=11&t=974&p=14328&hilit=Mithras#p14328, and find "Hartt").
In Egypt, as Plutarch recounts, the rising of Sothis, the dog-star, at dawn was taken as the herald of the Nile flood, thus an image of rebirth (http://thriceholy.net/Texts/Isis.html, sections 21, 38, 39). It was the beginning of the new year. On the other horizon, Aquarius was setting at the same time. Ptolemaic Greco-Egyptian zodiacs showed a man or woman pouring out two jugs for this sign. I don't know when these zodiac images would have reached Italy, but it may be significant that Aquarius was shown with two jugs only during the Renaissance, that I can find. Here are some examples from Egypt, one from a photo I took at Dendera, the others from Desroches-Noblecourt, Le Fabuleux Heritage de l'Egypte, p. 123. My information about Khnum is from Desroches-Noblecourt. I doubt if the Renaissance would have known the point. But they probably could have associated the two jugs with the two branches of the Nile.
http://1.bp.blogspot.com/_Lu-6PwakMv0/S ... ETNota.jpg
http://1.bp.blogspot.com/_Lu-6PwakMv0/S ... r2Nota.JPG
It is a mistake to think that Egyptomania started with de Gebelin. It was very much present in Northern Italy from the early 15th century until the Council of Trent, what with the discovery of several new texts purporting to decode the hieroglyphs, and also Egypt opening up to trade and tourism. It is described in detail in Curran's The Egyptian Renaissance: The Afterlife of Ancient Egypt in Early Modern Italy (http://www.amazon.com/Egyptian-Renaissa ... 0226128938). Both Pope Alexander VI and Emperor Maximilian VI had their genealogies traced back to Osiris, and each commissioned an appropriate Egyptianate portrait, the one by Pinturicchio and the other by Duerer. Ciriaco da Ancona went to Egypt three times, sketching as he went, then giving talks on his travels when he returned. He only got as far as the Cairo area, but that would have been enough to collect some interesting images, especially if he enlisted the help of some Egyptians. He spent his last days, c. 1450-1452, in Cremona, where the Bembo tarot was made around then. His sketches of Egyptian antiquities reportedly were acquired by Alessandro Sforza in Pesaro and unfortunately perished in two fires, according to Wikipedia (http://en.wikipedia.org/wiki/Ciriaco_de%27_Pizzicolli). Some of his Greek sketches are still extant.
You suggest that the water on the card could be the Nile. The Nile was known to have two main branches. One was the "White Nile", which picked up much clay and many nutrients in its slow course, but didn't have much volume. The other was the "Blue Nile", which during the summer rains in Ethiopia came down from the mountains in a torrent. This was described by Plutarch. If you look at the Cary Sheet card, you will notice a small hill on one side and a larger one, suggesting a mountain on the other. That seems to me perhaps to indicate the two branches of the Nile, one from hills and the other from mountains. It took both to produce the renewal of the land. It is not hard to draw an allegorical lesson from this fact. Here is the card, along with a photo of Roman-era Sothis:
http://3.bp.blogspot.com/_Lu-6PwakMv0/S ... Sothis.jpg
This is not to deny that the lady on the Cary Sheet card is also Venus, in which case the water would be the Euphrates, or in one version, the Nile. Venus and Cupid were said by pseudo-Hyginus to have escaped the monster Typhoeus by changing into fishes and jumping into the Euphrates; in Ovid, they jumped into the Nile or the Euphrates, and in the latter account two fishes carried them (all at http://www.theoi.com/Olympios/Aphrodite ... l#Typhoeus). I see two fishes on the card. There is also a star on her shoulder. It is another amalgam.
You ask about the seven stars as a constellation. The most logical to me, for the 17th century on, is the Pleiades, which in the King James Version, Job 38:31, was said (by God, no less) to exert "sweet influences" on humanity, i.e. exert astrological influence:
Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion?
However the Vulgate (more accurately?) seems not to have found this implication in the Hebrew:
31 numquid coniungere valebis micantes stellas Pliadis aut gyrum Arcturi poteris dissipare
(Shalt thou be able to join together the shining stars the Pleiades, or canst thou stop the turning about of Arcturus?)
But the two verses immediately after are about the morning and evening stars, so perhaps the association still stands:
32 numquid producis luciferum in tempore suo et vesperum super filios terrae consurgere facis
(Canst thou bring forth the day star in its time, and make the evening star to rise upon the children of the earth?)
In Greek and Egyptian mythology, which Job seems to follow, the morning star and evening star were separate entities, in Greece the masculine demigods Phosphorus and Hesperus. The name "Aphrodite" applying to both was a Hellenistic innovation, according to Wikipedia, I would guess conforming to longstanding Babylonian identification of the two with Astarte (http://en.wikipedia.org/wiki/Venus#Venus_symbol, section "Cultural Understandings").
There is much more allegorical meaning in the Star card. But that's enough on the points you raise.