Assorted questions

Not so distant, necessarily. This appears on the timeline some time between the Rebellion of the Inferiors (the first known public appearance of the Dark Eight, and therefore after the Reckoning of Hell) and the death of Orcus. If you go by the Forgotten Realms timeline, Orcus died only a decade or so ago (right after the Time of Troubles); Dead Gods has Orcus die centuries ago, however. The Reckoning was more than 1000 years ago going by Planescape sources (since Bel ascended more than 1000 years ago, and the Reckoning was before that), but as the previous rulers of Hell are still known, probably not much more than that. That could have been much more recently too, if you go by the 3rd edition assumption that the Hag Countess and Bel haven't even had time to put together proper courts yet. I tend to guesstimate the opening of Ghoresh Chasm at around 500 years ago.

Anyway, you probably don't care about that.

I've been actually meaning to write a post on Ghoresh Chasm for quite a while now, inspired by the city of Morglon-Daar from the 4e Demonomicon sourcebook. Basically, Morglon-Daar is a city of yugoloths built at the base of the Wasting Tower of Khin-Oin in a deep chasm. It's a 4th edition thing, so it combines two very different 2nd-3rd edition sites, Khin-Oin in the Gray Waste and the Grand Abyss in the Abyss. So in placing it in the Great Wheel cosmology, we have two choices. It could be placed in the Grand Abyss, as intended by the designers, and references to yugoloths could be toned down or removed; it's an interesting town regardless of what kinds of fiends rule it. Or it could be moved to the Gray Waste, at the base of Khin-Oin, and the chasm it's built in could be changed from the Grand Abyss to Ghoresh Chasm. Ghoresh Chasm could well be right next to Khin-Oin; it's said to be at the "center of the plane," if that means anything on an infinite expanse, and Khin-Oin is surely as central to the plane as anything else. If it's moved to the Waste, there needs to be some way to get from that point in Ghoresh Chasm to elsewhere in the planes, since it's meant to be a base for planar explorers. The Styx probably flows through it, pouring down the side of the chasm and into other planes and layers. It might well be the destination of the portal in Hopeless, with a portal in it that leads back to the Outlands. It might have other portals leading to locations in Gehenna and Carceri, or the Styx could be relied upon for that. Wrechtmirk in Acheron, too. And of course it attracts people needing to deal with the 'loths, and those curious about Ghoresh Chasm and the mysteries it holds.

On to Ghoresh Chasm itself. It's an enigma, of course, meant for the DM to fill in as needed. It's clearly something apocalyptic in scope, so it's a good place to plug in any plane-shaking Big Bad, buried beneath the Gray Waste for an eternity, only now awakening. It could be Tharizdun, as Kobold suggested (and Tharizdun is part of the background of the official Morglon-Daar), though I personally think Tharizdun is overused (he might not be overused in your own campaign).

I think Ghoresh Chasm is an opening to the original plane of evil from which the other lower planes grew. It could also contain an ancient race of evil older than any of the modern fiends, a baernaloth city (deep, deep beneath the chasm, far beneath Morglon-Daar or anything else that might be placed in it), the Far Realm, the plane of dreams, the Ordial Plane, a sleeping draeden, or anything else that preoccupies or interests you.

It might be something unexpected, like a buried plane of good long ago stolen by the forces of evil. Or a separate multiverse with radically different cosmological assumptions.

I think it hints strongly that she was blackmailed. My impression is that Emma wasn't really a nice person, and Rowan Darkwood probably felt he was perfectly justified in getting rid of her using her own crimes as weapons. In The Factol's Manifesto, Darkwood says he "told her a few things she ought to know," and I assume these were crimes of hers that he had learned.

Since fiends in 3rd edition lack the near-universal Teleport Without Error ability that 2nd edition ones had, then arguably yes, there was a permanent change.

Maybe. The Maeldur is itself a celestial, of course, so if celestials have the secret of true teleportation, the Maeldur presumedly gets its own powers from the same place that other celestials do. That means if celestials lost their ability, fiends would lose theirs.

The Maeldur works by knowing the true names of the beings it teleports, so apparently teleportation is a form of truename magic. Perhaps it involves transposing the names of places with other places of the same numerical value, in a form of planar gematria performed using the language of creation.

There are a lot of different things in the swamps of Belieren. The Hydra of Legend (called the Mother of Serpents in Todd "Shemeska the Marauder" Stewart's campaign) is one of them. The birthplace of the quesars is another, the primordial clay from which they were formed, and other things might be formed as well. There are also secret fortresses of the leonals, probably, and much else. There might also be imprisoned archfiends, portals to secret heavens and otherwise unknown planar layers, and the source of the multiverse's pain kept safe in the bosom of ultimate good. And yes, the source of celestial teleportation might be there too.

Aasimon are servants of the gods and therefore have little role in purely planar (as opposed to divine) affairs, except in Mount Celestia where aasimon and archons belong to the same hierarchy. The Hydra of Legend should definitely be under the control of the guardinals, since it is a matter for Elysium as a whole rather than the gods of Elysium. I don't know what you mean by "celestial overlord" - the rulers of the guardinals are Talisid and his companions.

The Angels/Aasimon are concerned with the good-aligned Upper Planes in general rather than just one plane, and they're somewhat the "overlords" if you will of the celestial races, much like the Yugoloths are the overlords of the fiends.
Also, while the Angels are indeed servants of the gods, they, like the other celestial races (at least if you go by the BoED) have their own agendas as well (which for the most part don't clash with those of the gods).

The concept that became the beasts of legend originated in the 1st edition Manual of the Planes, which noted that extra-strength monsters from Greek and elvish myth could be found in Olympus/Arvandor, and similarly enhanced monsters from Norse myth can be found on Gladsheim. That being the case, I think most of them ought to be found near the realms of the gods from the culture from which they originated. Of course, the hydra already breaks that guideline, since it's in Elysium rather than Olympus, but the guardinals brought it there from elsewhere.

Darn, then. But then how do you explain the Androsphinx of Legend on elemental Air? Shouldn't that thing be on Arcadia, where most of the Middleastern and central Asian pantheons reside?

Because monsters of legend are creatures tied to specific myth-cycles, you'll have to explain what they're doing in the Inner Planes, which is usually far from the mythoi that birthed them. Since monsters of legend are sometimes created for specific purposes, they might be sent there to fulfill them - but why in the Inner Planes in particular? I assume you're having gods and heroes put them in deep freeze in the Plane of Ice in order to keep them out of trouble?
No, I was naively unaware that they're mostly limited to the planes of the pantheons that birthed them (mostly due to the Androsphinx). I was assuming that all flagship magical beasts had Legendary forefathers, thus I was thinking about putting a Winter Wolf of legend on Ice, though since they're tied to giants they probably belong on Ysgard.
Not really sure where the giant raven of legend would go, though, as I think there is more than one culture with a giant raven (also, the giant eagle of legend could just as easily be of Amerindian myth, so how does one reconcile with that?)
Then of course there's reconciling with beasts of legend that belong to multiple pantheons. This becomes more of a problem with creatures found in both Persian and Hindu mythology, such as the Garuda.
It's less a problem with Egyptian vs. Sumerian vs. Babylonian since they're all located on Arcadia, but it does become a problem when the myths are shared between the Greeks, Persians, and Egyptians, whose pantheons are on nearly opposite upper planes.
It's also a problem with some creatures shared between Celtic and Norse/Saxon mythology.