There isn't one. So that's kinda huge. There are faerie realms, though. To reach them or leave them requires use of a Shadow Gate. They might be temporary or permanent, but these are the only routes in and out of Faerie.
Probably really don't need or want them. We could still have various courts if you wish. Even rotating leadership if you were really married to the idea. However we decide to divide them politically and whatever other personal agendas local leaders might have, there are still two major groups of fae: Seelie and Unseelie. Or, Summer Court and Winter Court if you like. Minor groups are far less numerous and certainly more splintered but they do exist as unaligned fae. These are the ones that typically get used and trampled by the more powerful courts.
We could create a new court or two, if someone has some burning ideas for them.
The True Fae
Eh, nobody calls them that here. Everyone's fae. If you got captured by a Sidhe Lord and escaped to the Mainland, then you aren't human anymore. You, my friend, are fae. Because no one ever, ever escapes Faerie. Not really. The Sidhe Lords are really that powerful and that scary.
See, the thing is, once you are no longer human at all, the Sidhe Lords (or Lords of the Sidhe, or heck, Lords of the Sith) just don't give a crap. Non humans are fairly useless for their purpose (unless they just enjoy having fae slaves, servants or soldiers). So, the only way to escape them is to stop being human. Therefore, no human ever escapes Faerie. Once you are all used up and therefore dead or have traded your soul to "escape", they just go out and grab another mortal. There's lots and lots of them. If you are a Sidhe with an especially twisted sense of humor, you might even drop the last mortal you used up in place of the one you kidnap. So what the mortals find is either a corpse, or someone that looks and talks like the person they knew, but isn't. They used to call these replaced people changelings, so that's what we'll call them here. Maybe that's your character and maybe it's not. You aren't restricted here in terms of where you come from.
Even lesser fae might kidnap mortals. They do it for the very same reasons the Lords of the Sidhe do it. Or they might do it because some Sidhe Lord has commanded them to do so. They might not be personally as powerful as the Sidhe Lords, but they are just as evil.
Never happened. Move along. (Or we can compromise. Perhaps they happened, but were sealed or sealed themselves after a time instead of just forever getting worse until they swallow Mythic City.)
Differences Between Mythic and Other Shadowgate Games
The main difference is that there is not Shadowgate council. In the original concept for the setting, there is a group that maintains order between the various supernatural groups and that will step in if things get out of hand. This isn't present for Mythic.
But it could be, if the players think that might be an interesting element. Or, perhaps it might form during the campaign in response to player actions.