Perhaps, but I think the part leading to Sirona's downfall would need a little more explaining. At the very least, she should have some assurance of the safety of her people--a promise that would be broken, or interpreted much differently than she understood it.
I'm not sure why, but I seem to have the impression that perhaps the Godslayer must be wielded by a mortal. It just seems appropriate that she fall by mortal hands, perhaps slain by Tebyrn, or even one of her own followers who she was trying to save who was tricked or converted to the dark side. It might be appropriate for her to risk her life for the sake of the repentant Sheaim, and then die at the hands of one who wasn't as repentant as it seemed.
Perhaps she tried to reason with Agares, but the other good gods had other plans. They led a champion wielding the godslayer to their meeting spot to slay the King of Hope, but when they failed the blade was turned on the innocent Sirona.
I really don't see the Sidar turning to use Death magic, which they abhor. They revered Arawn, so once they found out who Laroth was and what he had done they would turn against him. Futilely, of course.
I get the impression that if Laroth managed to become a god he would not stop at dethroning Arawn. Sirona's precept would be vacant then too, and he as the greatest master of Spirit magic would consider himself her natural successor. As a former mortal controlling one precept would leave him the weakest of the gods, but if he controlled two, well, thats a different story. The other gods would have to accept him as a equal then.
Being a god of 2 precepts can cause them to merge, and make neither be served as well as it should. Sucellus gave the precept of Nature to Cernunnos to prevent this, since as a good god he did not want the precepts to become something other than that which The One had intended. Laroth, however, clearly does not care about this. Thus, the spheres could begin to merge in to one, with their convergence perhaps best represented in suicide. I imagine Lethe might join with Laroth, perhaps becoming his archangel.
The corruption of the precept of spirit would cause the corruption of some Elohim, as I described earlier. They were not close to Laroth himself, just his new sphere, so not all of them would be corrupted, just a fragment. This fragment may lie undiscovered for perhaps the whole age, but as an unknown enemy within the gates of the rightous they are among the most dangerous.