They're both explicitly presented as First Age elders in their individual write-ups.

The world is full of supernatural entities whose presence and activities aren't sanctioned by the Immaculate Philosophy, but which aren't necessarily so troublesome to the Perfected Hierarchy as to warrant extermination. An Exigent or other unidentified Exalt should be reported to a monk, but only those who bear the signs of Lunar or Solar Exaltation are innately considered to be,Anathema.

Basically, by default Exigents (like god-blooded) and other non-Dragon-Blooded Exalted are seen by the Immaculates as the result of inappropriate congress between gods and mortals, but can atone for their inauspicious creation through noble service. But they have no innate role in the well-being of creation; they can easily go astray, and those whose evil deeds draw sufficient condemnation may be dubbed Anathema on an individual basis.

Yes, along the lines of the ones found in First Edition books like Exalted: the Dragon-Blooded and Scavenger Sons.

Occasionally mentioned in passing where relevant, but it receives no dedicated coverage.

Discussed in the Immaculate Order chapter.

There's a full chapter on satrapies, including both general discussion of the satrapial system and individual write-ups of several satrapies.

Page 32, "The Realm Historian Speaks," largely carries over in its broad strokes, though it's worth noting that it's explicitly presented as a systematized compilation from various sources rather than a universal historical orthodoxy. (One noteworthy change is that Sidereals are no longer considered Anathema by default in 3e.)

As Vance says, in 3e "Akuma" isn't a mechanical template that you paste onto an Exalt. It's an umbrella term for someone who's been granted infernal power. The term "Terrestrial Exalted Akuma," by itself, provides no meaningful information on the particulars of the infernal powers that a Dragon-Blood may have received. So the answer is that whether any given akuma is a challenge to your PCs is up to whatever traits your Storyteller decided to assign to that akuma.

(This seems like something we might want to provide guidance for in a future book. But it's not something that we have the wherewithal to address at this time.)

We're aiming to cover these things in an upcoming book about spirits, if possible.

Some demonic omens are more dramatic in their impact than others, but by and large, they aren't meant to unexpectedly murder your PC's familiar.

Any artifact rated three dots or higher is theoretically capable of developing Evocations,

For both of these questions, it depends on the nature of the specific airship. A two-dot hot air balloon whose sole magic is a self-healing canopy will have very different mechanics from a five-dot blue jade aerial cruiser with integrated lightning ballistae.

There's no inherent rule against it, but the combat advantages of Legendary Size are significant; you'll want to be careful not to make a Legendary Size demon of the First Circle into the optimal combat summons.

The Golden Hounds (and other unique artifacts in Arms of the Chosen) are highly unlikely to reappear elsewhere; that would undercut the flexibility of providing artifacts for your PC to wield. Admiral Sand is not appearing in Heirs of the Shogunate.

A legion has 5,000 legionnaires, roughly 2,000-3,000 skirmishers, and a small assortment of officers and specialists. Auxiliary forces attached to legions in the field vary widely in size and composition.

What you're missing here is that satrapial garrisons aren't composed of legionary forces. The garrisons have always been supplied by the Great Houses, even before the Imperial legions were divvied up among the houses after the Empress' disappearance. They are a completely different set of troops.

That's up to you!

Cynis Denovah Avaku wasn't chosen as a backer character for Heirs to the Shogunate.

At present, no five-dot artifacts but several four-dot artifacts. Specific dot ratings may be subject to change during development, however.

None. We didn't overwrite for What Fire Has Wrought; everything in Heirs to the Shogunate is new material.

I don't think it's likely to come up in the text one way or the other.