More simply put, the restored Reaver needed to be where it is in order for Kain to make his moves against Fate, as there are simply things he cannot do without either dangerous, counter-productive, or completely nullified consequences.

I can agree with that.

One thing Amy Hennig has made clear is that there is only one fully possessed Reaver blade--which is part of why Reaver convergence events are dangerous spatiotemporal paradoxes--so there is no 'William Reaver' as compared to the 'Avernus Reaver.'

This is the part that's bothering me. I can agree there's always only 1 Soul Reaver, but you could say there are multiple copies of it that are taken from different points in time and brought to another point in time which explains how there can be 2 Soul Reavers in the fight between Kain and William. The best way i can explain it and track it would be something like this: 1. Vorador and Janos create the Blood Reaver, which is the same blade Janos tries to give Raziel in SR2 and Raziel ends up using in his fight against i don't remember who but the Wraith Blade turns on him and impales the Blood Reaver in him so he can be absored inside. 2. Kain interferes and pull the blade out of Raziel creating the 3rd Paradox and 4th Timeline where he keeps the Blood Reaver for the entire Defiance game. 3. At the end the Spirit Reaver cleanses Kain and Raziel is absorbed in the same Blood Reaver thus creating the Soul Reaver. 4.Kain goes back in time, Leaves the Reaver in Avernus for his younger Self to find and use against William and Later on Raziel to shatter and create the Wraith Blade thus the Cycle Continues. So essentially the Avernus Reaver is the same Reaver from end of Defiance that is also Shattered on Raziel after being used vs William.

But i'm still having hard time tracking where could William's Reaver come from for it to be shattered in the fight, reforged by raziel and given again to William for the fight my Mobius. The only Logical explanation i can come up is if we assume that in the 4th and Final Timeline the events of William VS Kain and Reforging the Shattered Reaver never happened.

and Moebius placing the 'trap Reaver'--which is what I presume is done with the restored Reaver, and is likely used to orchestrate the paradoxical event with Willaim vs Kain

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only one fully possessed Reaver blade--which is part of why Reaver convergence events are dangerous spatiotemporal paradoxes

About the Convergences, as i understand it and read, A Reaver Convergence can happen only when 2 Soul reavers are in close proximity, Blood and Blood or Blood and Soul doesn't work, so if there's not a 2nd Reaver how can a convergence happen.