I'm gonna make a post here, but mainly because it's become a repository for the Waterdeep campaign. If you want to comment on it, feel free, but it mainly concerns world history that will never be known by the PCs.
Because the spell weaver Ma'kar was central in Kyuss's ascension, it is important to explain his motives and designs. In trying to do this, I read all of the little lore there was on spell weavers. One of the few things that is known is that they habitually steal magic items. They only give magic items away when the item is likely to ruin the wielder and his civilization. Meanwhile, Ma'kar gave Kyuss the first Kyuss worm. The description of Ma'kar's interaction with Kyuss was that it was an exceptional case where a spell weaver was not acting ethnocentrically but was helping another rise to power. This absurd exception begs for an explanation. Then it suddenly occurred to me, that for any intelligent victim, a spell weaver who gives him a ruinous item is also going to have to give him an "official story" that convinces the victim that the gift will not ruin them. That story is what's recorded in the lore. But of course what really happened is that Ma'kar was acting in perfect accord with his species. The gift of the Kyuss worm would not only ruin Kyuss's civilization, but also every other civilization, including the Illithids', when Ma'kar's plan finally came to fruition.
But there's still more that needs explaining. In reading the spell weaver lore, I came across something called the Code of Revision. The spell weaver civilization itself was ruined ages ago in an unknown experiment that ended in catastrophe, as was their ability to use the Code of Revision, which apparently was some time travel ability. While the code is described as being designed as a fail-safe against the cataclysm, it never rules out (and I assert) that while it could be used specifically to revert that event, it was used before that on a regular basis to basically reset the clock on the spell weaver civilization. Kind of like a Narnian wardrobe, the spell weaver civilization could study magic and experiment for millennia but only take up decades in the world's timeline. But according to the lore, spell weaver time travel is particularly nefarious. The original timeline is annihilated, leaving only the destination timeline in tact. But I'd like to go further and fill in some gaps in the lore. Counterpart beings in the destination timeline are switched to the original timeline right before it's annihilated, thus annihilating them as well. In other words, when George the spell weaver goes into a past when he existed, then younger George is actually switched with older, time-traveling George. Older George then is in the past while newer George has been switched to the future where he and it are annihilated. This is also a convenient explanation of how the Amulet of Second Chances works. And as a spell weaver time-travel artifact, access to it should appropriately be severely limited.
Now, as for the civilization-ending cataclysm, I'm thinking the spell weavers tried to divorce Mystra from the Weave. I've always thought if I were a mage in a fictional universe, I would be rather disappointed if there was a god of magic. The reason of course is that if there's a god OF magic then there's a god OVER magic and that means that mages are beholden to a god, which is, in my mind, exactly what mages shouldn't be. So, if I were the ultimate magical being in the Forgotten Realms, a spell weaver, I would desire that the Weave functioned independently of Mystra. Of course, I could imagine all sorts of reasons why an attempt at such a divorce could fail catastrophically. So we now have the reason why the spell weaver civilization collapsed.