Well, first good work with that.
(I want to say that I'm new in this, so there's a high chance that I'm wrong)
I always assumed that the breaking of the Oathpact was linked directly to the Recreance. How could the orders keep existing if their leaders had left, had betrayed them? if there would be no longer Desolations? what would be their porpuse?
Now, I will only say that the Heralds deciding to leave the Oathpact falls right in place with each epigraph (I think). Also, the fact that the Radiants left their plate and blades means something. Their spren died but remained in physical form? that seams fishy. Maybe they didn't break their oaths but the act of the Heralds leaving the Oathpact might have, or at least it prompted it.
So, in short I think the matter of "why" the Recreance happened is the breaking of the Oathpact by the Heralds.
I repeat, I'm just sharing my opinion. And I realise that the idea is simple and that I don't have much to back it up, but well I remember taking this as a fact while reading.
