There is much more confusion over Alexander's birth date than it seems. Yes, Plutarch (Alx.3) does state that it was 6 Hekatombaion which he equates to the Macedonian month of Loios and Aelian
Book 2.25
They say that the sixth of Thargelion brought much good fortune not only to Athens but to many other cities. It was for instance the date of Socrates’ birth; the Persians were defeated on that day; on it the Athenians sacrifice to the goddess Agrotera three hundred goats, acting in accordance with Miltiades’ vow. The sixth day at the beginning of the month is also said to be the date of the battle of Plataea, when the Greeks were victorious. The previous defeat of the Persians, which I have mentioned, was at Artemisium. The Hellenic victory at Mycale is also accepted as having been the gift of that day and no other, assuming that the victories of Plataea and Mycale were on the same day. Alexander of Macedon, son of Philip, is also reported to have crushed the many myriads of barbarians on the sixth of the month; that was when Alexander defeated Darius And it is believed that Alexander himself was born and departed this life on the same day.
Which can be understood as meaning that Alexander was born on the sixth of a month not necessarily Thargelion (May/June).
BUT Aristoboulos Arr.VII 28 i says Alexander lived thirty-two years and eight months and reigned for twelve years and the same eight months. Since he died on the last day of Daisios 323 that would mean he was born late in Hyperberetaios or the beginning of Dios (Octoberish).
A letter of Philip's of 339 preserved in Demosthenes De Corona 157 equates Loios of 339 with Boedromion, which would require a slippage of about thirty seven days (assuming that the date in Hekatobaion is really the second sixth, waning actually the 25th) between the Athenian and Macedonian year in a period of only seventeen years which seems too much. It may be that Plutarch means that in his day Loios was equivalent to Hekatombaion.
It must be better to go with Aristoboulos.
For reference the Athenian year ran the Macedonian
1 Hekatombaion 10.Loios
2 Metageitnion 11 Gorpiaios
3 Boedromion 12 Hyperberetaios
4 Pyanepsion 1 Dios
5 Maimakterian 2 Apellaios
6 Poseidon 3 Audunaios
7 Gamelion 4 Peritios
8 Anthesterion 5 Dystros
9 Elaphebolion 6 Xandikos
10 Mounichion 7 Artemisios
11 Thargelion 8 Daisios
12 Skirophorion 9 Panemos.
Plutarch and Philip might be reconcilable by making 356 a 12 month year and 340 a 13 month one in Macedon, the additional Xandikos Embolimos would push the Loios of 339 to Metageitnion requiring less of a slippage but nothing is going to explain Aristoboulos' implied birth date in Hyperberetaios/Dios.
When you think about, it free-choice is the only possible option.
Anyway, yes, Hestia was sometimes included in the Olympians although I'm not sure who she replaced. And Hades wasn't included in the twelve (he didn't live on Olympia), but Dionysos. There were plenty of other lesser "Olympian" gods also - Theoi.com is the place to go for info. Kind of like Pothos for the Greek gods only without a forum.
