Need some context on a few things
There isn’t any single great place, but running the things you have questions about through the wiki is probably your best bet to start.
To answer these two, though- the centaurs are the immediate, kingdom-wide threat. The Elder Dragons are the longer term, world-wide threat. Their gig is basically to slowly corrupt the land, and the creatures, and the people, and at your part in the Personal Story they’re thought to be pretty much invincible. They haven’t bothered Kryta for some time, but when they do turn that way, they’ll be a much bigger threat than centaurs. Unless they’re stopped first, which is what the orders are all about.
As for Destiny’s Edge, they were the only guild to make any substantial victories against the dragons in what seems to be a long time. They got famous for that, they let it go to their heads to some extent, some split loyalties came up at a very inopportune time, they lost a fight, one of them died, and the rest all split up blaming each other. Again, the wiki would be a decent place to go, as long as you don’t mind spoilers.
(edited by Aaron Ansari.1604)
1. The human race is directly threatened by Centaurs and Bandits (which include ministry corruption and White Mantle actions). It is common knowledge that the Elder Dragons are a thing, but they are very distant (akin to how someone living in Miami would view ISIS kinda). For the common people, it is more important to deal with what’s at our doorstep rather than some distant threat that might never impact them directly.
2. Destiny’s Edge is a small guild that was successful in slaying Dragon Champions. Each of them is an influential member of their race, or has a lot of respect among their peers. They disbanded because when they were about to fight Kralkatorrik (the Dragon that left the black/purple scar on the land of the Charr called The Brand), Logan left the rest of the group alone to go save Queen Jennah in Ebonhawk, which resulted in the death of Snaff (Zojja’s mentor) and Zojja blames Eir for her lack of leadership, Eir blames herself for the death of her comrade and useless death of Glint, and Rytlock blames Logan. Much drama ensues, and they disbanded. (read Edge of Destiny for more detail)
On #1, if you pay attention to the various details, it turns out that the centaurs are not actually Kryta’s main threat. The bandits and their political backers are. The centaurs have only been having the success they’ve been having due to Kryta being sabotaged from within.
It’s a bit like the Star Wars prequels: the centaurs are basically taking the place of the Separatists, while a Palpatine is on the inside pulling strings trying to get into power. The distinction being that Jennah and the Shining Blade are more politically astute than the Jedi Council – they know what’s going on and have been actively fighting against it. However, while they know what’s going on, up to now they haven’t been able to get enough evidence to move openly against the perpetrators.
People don’t hate Scarlet like Game of Thrones fans hate Joffrey.
They hate her the way Star Wars fans hate Jar Jar Binks.
Regarding centaurs and dragons – the whole dragon threat is told more through the open world across the entirety of it, for players, but is treated as common knowledge (more or less) for characters. None of them have an exact location (except those we’ve fought), just a general direction – Jormag’s in the north, Kralkatorrik was last seen in the Crystal Desert, Primordus is underground, etc.
Every race has a “direct” threat and the Elder Dragons all serve as a distant one. Centaurs and bandits (aka White Mantle front) is the direct threat for humans. Its their day-to-day problems.
Regarding Destiny’s Edge – do the dungeon story modes. That focuses on them, and does cover who they were and what they did and why they broke up. The novel Edge of Destiny explains it in even more detail, though the writing isn’t the best.
Stop treating GW2 as a single story. Each Season and expansion should be their own story.