4. If Celaeno should be at summer solstice and Alcyone at winter solstice, then the 2nd half of the year should lie between them. Beyond Alcyone would then come Atlas and Pleione, with Atlas upholding the sky and Pleione promising a year of plenty.

Who were these Pleiades sisters? I have extracted a few facts from the description in the site ianridpath.com:

"... To a casual glance, the Pleiades cluster appears as a fuzzy patch like a swarm of flies over the back of the bull ...

According to mythology, Alcyone and Celaeno were both seduced by Poseidon. Maia, the eldest and most beautiful of the sisters, was seduced by Zeus and gave birth to Hermes; she later became foster-mother to Arcas, son of Zeus and Callisto. Zeus also seduced two others of the Pleiades: Electra, who gave birth to Dardanus, the founder of Troy; and Taygete, who gave birth to Lacedaemon, founder of Sparta. Asterope was ravished by Ares and became mother of Oenomaus, king of Pisa, near Olympia, who features in the legend of Auriga. Hence six Pleiades became paramours of the gods. Only Merope married a mortal, Sisyphus, a notorious trickster who was subsequently condemned to roll a stone eternally up a hill ...

Eratosthenes says that Merope was the faint Pleiad because she was the only one who married a mortal. Hyginus and Ovid also recount this story, giving her shame as the reason for her faintness, but both add another candidate: Electra, who could not bear to see the fall of Troy, which had been founded by her son Dardanus. Hyginus says that, moved by grief, she left the Pleiades altogether, but Ovid says that she merely covered her eyes with her hand ..."

Merope is special and the reason, I believe, is her postion below the 'water line' between Electra and Alcyone:

Gods are fiery creatures in the sky and they cannot be down in the 'sea'. Therefore Merope had to marry a mortal. And, of course, a stone must be part of the story.

Maybe only 6 of the sisters should be counted.