a pantheon is temple that is dedicated to not one god, but all of the gods
There are well-known Pantheons in Paris and Rome.
a lot older now than it did back when it was made
The Greek goddess Persephone (Roman Proserpine) is probably the best known goddess of the underworld. Other pantheons have different goddesses.
A pantheon is a temple to all the gods, a good example is the one in Rome. Is there a pantheon in Athens, I've never come across it.
Yes, Wicca does exist. Wicca is a pagan revivalist religion that incorporates many ancient Celtic traditions with other pagan pantheons.
The Norse had no name for Hera. The Norse and Greek pantheons are vastly different, and there is no direct correlation between the two, as there are in Greek and Roman.
Celtic Mythology was developed independent and indigineous to the British Isles, though it bears some similarity with other Northern European Pantheons.
Wiccans use several Pantheons depending on their traditions. My best suggestion is to check with the specific group you are interested in. But of course, All of the gods and all of the goddesses, are only aspects of the goddess and god.
The head of the Greek and Roman pantheons is Zeus also known as Jupiter or Jove. It is believed he is the same god, as the Greeks and Romans frequently shared culture, be it by free trade or conquest.
Aztec religion did not have a hierarchy of sorts for its gods, as did some European pantheons. However, Huitzilopochtli is considered the patron god of Aztecs and was as such accorded a high status in the holy cities.
In general terms - Nature. However each pagan will have their own path, some worship the God/Goddess - others will worship specific deities from the old pantheons. It's very much a personal choice.
Of course you can. In fact, the Dieties and Demigodshandbook gives you a complete combat profile for all of the gods in the D&D, Olympic, Norse, and Egyptian Pantheons. Most of them can be easily handled at epic levels.