2 Books to go. What back story do you want explained?

[Vraith]

So is it possible that the Worm's awakening is and always has been a part of the Elohim Wurd?

The eventual awakening wasn't part of the Elohim's Wurd, I don't think: I think it was definitely [probably primarily] the thing the Wurd was meant to oppose [for as long as it could]. Kinda like a healthy lifestyle isn't going to prevent death...just hopefully put it off, give the longest, most livable/meaningful life possible.

[Horrim Carabal]

The knowing sacrifice made by Findail and the Colossus Elohim seems to be made at the point of accepting what they are to become.

However, the same doesn't appear to be true for Kastenessen. He clearly can change his mind

Maybe he was flawed in some way, unable to accept his fate? Maybe he is stronger of will than the other Elohim? Maybe he was already not quite sane, unbeknownst to his kin?

Any way you slice it, Kastenessen was a huge gift to Lord Foul. Where would he have found another tool of such value?!

[bikebryan]

I'd forgotten that one - that's very true. One wonders how much free will (if any) that Elohim had at any time, once having been appointed as Guardian of the One Tree.

The fact that the Theomach as an Insequent could defeat an Elohim, thus becoming Kenaustin Ardenol, is interesting in itself, given the much-demonstrated powers of the latter. I suspect that the Elohim accepted becoming lessened in some way in being appointed as Guardian. Or alternatively its defeat was a necessary part of its Wurd.

The Theomach did not "become Kenaustin Aredenol" because he beat the Elohim guarding the One Tree. He was Kenaustin Ardenol long before then, but did not reveal his true name due to the power it gave over him.