Different mythologies being consolidated, mainly.
Originally—by which I mean as far back as we have evidence to speculate—Poseidon seems to have been married to the earth-deity Dā. The oldest attested form of his name is Maecenean po-te-da-on, presumably from potei Dāōn, "husband of Dā"; this would make him the son-in-law of Demeter (Dā-mātēr, "Dā's mother")...
"Solve et coagula" is the principle underlying alchemy: dissolve materials to their constituents and re-assemble these into something else.
Fulcanelli writes of this quote:
If you know how to dissolve the fixed,
And to make the dissolved fly,
Then to fix the flying in powder,
You have something to console yourself with.
To answer your first question yes, Hermes was (among other things) a god of thievery.
An excerpt from the Hermes Theoi page
HERMES was the Olympian god of herds and flocks, travellers and hospitality, roads and trade, thievery and cunning, heralds and diplomacy, language and writing, athletic contests and gymnasiums, astronomy and astrology. He was the ...
A Euhemerist reading of the widespread Heracles cult was attributed to
a historical figure who had been offered cult status after his death.
Thus Eusebius, Preparation of the Gospel (10.12), reported that
Clement could offer historical dates for Hercules as a king in Argos:
"from the reign of Hercules in Argos to the deification of ...
This probably derives from the Greek coinage spagyria, which itself is supposed to come from 'σπάω' and 'ἀγείρω.' These words roughly correspond to "solve" and "coagula," or divide and join.
This word was used by Paracelsus, perhaps the most famous alchemist. I am using Andrew Weeks' translation, which I believe is from the (original?) German text.
Four years, according to the introductory plot summary of Seneca's play Phaedra, is the amount of time Theseus was in the Underworld.
The Mechanics Thereof
I think you've answered your own concern regarding how it is Theseus and Peirithous were able to remain alive while they were trapped in the Underworld. Considering the powers possessed by the gods and ...
One More Wife
To complicate the issue yet a little bit further, if we take Plato's dialogue Kritias into account, Poseidon actually has three wives, not just two. According to the description of the foundation of Atlantis in Kritias, Poseidon was married to a certain Kleïto [Cleïto] who bore him five sets of twin sons who ruled the land, which was named ...
Lechia is the mythological name for Poland, stemming from the word Lech (which is also a common first name).
According to legend, the name derives from the first ruler of Poland, Lech. It is also the derivation for the term Lechitic languages.