It's hard for me to quickly present everything i'd like to ask about/discuss without writing my own novel here, but i'm going to attempt to be succinct. It doesn't help that a lot of this info is sorta haphazardly scattered over Q&A sessions. Hopefully not too much of this is already known info(didn't seem to be much discussion on it at the very least?)
Most of my information stems from the 2 letters, but to start, my very small understanding of the timeline:
First there was hoid and maybe dragons(i'm assuming one wrote the second letter) and Yolen
At some point there was Adonalsium, and then it/he/she split.
By this point at the very latest humans have spread from Yolen to other worlds(which are both physical planets and possible alternate dimensions? Can't really tell if all these places are just physically distant or dimensionally sepereate)
Sometime after this Odium visits Sel, and kils two shards.
Sometime after that, Odium attacks Roshar(from Braize?) for, what seems to be, several thousand years. He fights at least 2 more shards here, and at some point(after the Last Desolation) kills at least one of them. Odium becomes trapped here at some point.
By this point at the latest humans are kicked out of the Tranqualine halls(I suspect it's MUCH earlier).
4500 years after that WoK starts
Somewhere in the above Harmony becomes a thing and starts playing the shard/god game. Just before this is WoA where Hoid steals the metal, which implies his quest against Odium has been going on for at least this long(although likely much longer). The theft seems to be what prompts some other form of contact predating the first letter.
Also at some point the dragon either creates, or uses the 17th shard(which at the very least contains recent characters, but could fall anywhere in the timeline for creation), and given some of the wording in the first letter, the dragon becomes essentially immortal, rather than actually having always been that way.
So if my understanding is correct, this seems to confirm a few things so far:
Adonalsium, whatever it was, doesn't seem to be the "let there be light" level of god/creator. While it may have created worlds, the fact that worlds predate it(and possibly humanity itself) along with beings like Hoid implies there may actually be stronger entities out there. In fact it seems to heavily imply that Adonalsium was either created, or "occured" after humanity? This could all be misinterrpreting a quote though, as I know there's a short story of hoid from the early early early days planned, but I don't know if that actually takes place before the existance of Adonalsium, or just the shattering, or neither.
That said, a little bit of further evidence seems to be in the second letter with the quote "The worlds you now tread bear the touch and design of Adonalsium." Maybe it's just me but this seems to possibly imply that there are worlds out there that do NOT bear the touch and design of Adonalsium, and that Hoid has traveled those as well.
Either way it seems that Hoid's main goal is to stop Odium(although likely not for purely altruistic reasons). This is interesting because the main point of the second letter is that while Odium is certianly a threat, he should be left alone because he can't exit the system he currently inhabits(Braize). For whatever reason the Dragon seems to believe that Hoid is going to cause more damage than Odium can, and that Odium cannot escape.
This is especially interesting because we don't know much about shard travel, but i'm not sure if Odium made a planet hellish and got stuck on it, or was stuck in a hellish like planet. Heck the planet itself could be anything in the physical world, and it's the spiritual world of the planet that's hellish(as that's the only description we seem to have). Since we know shards exist in 3 forms, i'm assuming that at the very least Odium's physical form is trapped there. Seeing as how he's the one who is behind voidbringers, and they're the ones who threw humans out of the Tranqueline halls, I have to wonder if Odium's plan involves escaping through the spiritual realm to continue his slaughter, but the more important question is who trapped him in the first place. The obvious answer would be Honor, Cultivation, and a possible 3rd. The heralds being sent to damnation has some similarities to the whole Ruin/Persevation thing.
The final piece to all of this that's been bugging me is Hoid's reaction to shardblades. It's possible he's just so powerful he doesn't care, but I've always assumed they harm people on a spiritual level(given the whole severed soul thing), and perhaps this also relates to why Odium seems to somehow be heavily involved in the spiritual realm.
Other random questions:
Where are the tranqualine halls? The system Roshar is in seems to have only 3 planets, and while one is their actual world, and the other is literally their damnation, the 3rd planet doesn't seem to be related to the Roshar humans at all(setting for a seperate book). I find it odd that their hell physically exists but their heaven does not. Especially given that while there's vast differences between the cognitive and the physical world, there does still seem to be some similarity, and with Braize being right there showing that the spiritual world can line up, one would expect the 3rd planet to be the Halls.
Again i aplogoize for the rambling. There's so much to connect/go over, and it's difficult to throw it all together without getting horribly sidetracked and rambling off on tangents. To summarize some of the main ones:
Why does Hoid think Odium can still escape?
How is Odium trying to escape?
How long has he been stuck there?
Why is he stuck there?
Whats up with the spiritual world anyways?