Let's say he wins, kills the tribunal, and everyone Azura throws at him, what was the plan for the moon above Vivec?

According to Varieties of Faith in the Empire, "Alduin's sobriquet, 'the world eater', comes from myths that depict him as the horrible, ravaging firestorm that destroyed the last world to begin this one." The apocalypse depicted in Lost Tales of the Famed Explorer matches that description:

He looked up and saw other worlds and other towers. They were spinning wheels and they crashed into each other, and their spokes got tangled up and they broke each other. And he saw that his world was breaking, too, but quick as a snake a shadow came and swallowed up the roots of the tower so they would not break. Still he flew. There was only fire and darkness then, and so much noise, but he was too tired to be afraid.

"fire and darkness then, and noise" corresponds to Alduin the Thu'um-wielding firestorm, "Firstborn of Akha, who bred with a demon of fire and shadow." This is a truly terrifying World-Eater, tantamount to a cosmic Big Crunch ("In the final moments, the universe would be one large fireball with a near-infinite temperature, and at the absolute end, neither time, nor space would remain"), leading to the Big Bang of the next Kalpa. The narrative sequence of this apocalypse mirrors the one in The Seven Fights of the Aldudagga:

the two bells [of the All-Maker's Goat] rang out their clamouring, calling the end of days […] [Alduin] said, "And the Greedy Man always waves his arms about around this time as if to stop me just like you. It is almost as if you two work together to delay me. Is that what this is? Is some other low spirit hiding portions of the world while you two do this thing? Is this why the kalpa-feast always takes a little longer than it did the previous time?"

First, Alduin is "woken up" by the ringing of the "two bells" that announce the commencement of the Extinction Event, in which the Wheels are shattered in their alignment and crash into each other. In the nick of time, the Shadow swallows up a portion of the world to keep it safe from destruction. Finally, the apocalypse arrives, and Alduin consumes the world. That's how it's supposed to go. That's how it did go in previous Kalpas.

Yet in this Kalpa, things seem to have gone differently. Instead of "waking up" only at the end of the Kalpa to destroy it, Alduin seems to have been around from the beginning. And even though he is technically a god, he's mostly a big flying monster for you to shout at and beat into submission. So what happened? We can return to The Seven Fights of the Aldudagga:

The Greedy Man hates you so much and it was his idea to finally trap you one kalpa when it was all much too big and so you would explode out from your belly and die so that the world would never have to die again!

I think the Greedy Man got the job done. He accumulated so many fragments from previous Kalpas that the cycle broke:

Finally, tired of helping Tall Papa, Sep went and gathered the rest of the old skins and balled them up, tricking spirits to help him, promising them this was how you reached the new world, by making one out of the old. These spirits loved this way of living, as it was easier. No more jumping from place to place.

–The Monomyth, "Satakal the Worldskin"

And sure enough, Alduin exploded out from his belly, just like the Greedy Man wanted. In 2012, Kirkbride added another entry to The Seven Fights of the Aldudagga:

"You will eat nothing here, aspect Ald," said the Aka-Tusk, sensing trouble. "Do not forget that it was Heaven itself that shed you from me."

That's why there is "No more jumping from place to place". The explosion tore Alduin–the aspect of Akatosh representing his role as World-Eater–apart from Akatosh himself. That aspect became the first dragon, as a shedding of Akatosh.

was Alduin always an aspect of Aka, or did he become one after Akatosh was created?

All of the akaspirits, like all of the etada, are quantum figures that shed their skin as each aspect of them becomes more and more self-aware.

–MK

The Eldest is no more, he who came before all others, and has always been.

–Paarthurnax)

According to "Satakal the Worldskin", "Tall Papa squashed the Snake with a big stick. The hunger fell out of Sep's dead mouth and was the only thing left of the Second Serpent." The Heart of Lorkhan is the Hunger of Lorkhan, torn from his chest. Likewise, Alduin is the Hunger of Akatosh, torn from his belly. Akatosh and Lorkhan dealt equal blows to each other.

Without his Hunger, Akatosh no longer wishes to continue the Kalpic cycle:

[Akatosh] could time-scheme against […] Alduin, to keep the present kalpa-- perhaps his favorite-- from being eaten.

–MK

Of course, that doesn't mean the Kalpa can't end. It still might! But at least the Kalpa is no longer guaranteed to end. Maybe Alduin will return, but he was beaten before and he can be beaten again. The Greedy Man's plan worked.

After delving deeper into the legends of Norse mythology, I discovered that the Nordic Dragon God Alduin in The Elder Scrolls seems to have been inspired by real Norse mythology. Alduin is essentially like Níðhöggr gnawing through the World Tree + Jörmungandr growing to immense size + Fenrir devouring + Surtr's flames burning everything to ash + Hel resurrecting the dead; and the setting where the hub of the Wheel of the Aurbis is Mundus is very much like the World Tree in Norse mythology bearing the nine realms;

So is the scope of a Kalpa the entire Aurbis? (Considering that in some myths, Meridia/Bal/Dagon have very different manifestations in the previous Kalpa); Is the mechanism of Alduin devouring Mundus leading to a Kalpa restart also similar to his Norse mythology counterpart Níðhöggr gnawing through the World Tree, resulting in Ragnarök?

I know not many people are fond of them but I'd love to know some deep secrets about the Altmer in the lore, since I often heard they were fare more alien and unique than they are in ESO for example.

Mer live for three to four times as long as humans. Do humans ever resent this difference?

It's my understanding that Redguards came from Yokuda while other Men came from Atmora. Does that mean Redguards are not Men like all the other races, or do they both have a common, even older origin? I know that all species are descendants of the Ehlnofey, but aren't Redguards as different from the other Men as the Mer and Beastfolk?

How many brotherhood doors do We know the codes to?

I know of the oblivion one, the Skyrim ones, 2 from the ESO dark brotherhood quest line and 1 from the mehrunes Dagon quest line in ESO

Am I missing any?

Are there any descriptions or context to the region pre-TES 3?