While certainly the implication is that the WoL is reincarnated piece of the 14 Convocation's soul, I've also been entertaining the theory that Minfillia/Ryne actually are the holder of that spot. Minfillia always seemed to have a deeper connection to Hydaelyn than even the WoL and her role as Word of the Mother would seem to be one to be held by the one who came up with the idea to summon Her.

Another reason I've been entertaining this idea is due to the battle with Hades, specifically when we break free from the chains he puts on us near the end and Light swells up briefly. I've had two theories as to what this specifically represents.

1.) The more likely one IMO, is that Hydaelyn intervened at this specific point in our favor to give us just the edge we needed to end the fight. Of course she's still greatly weakened thus why the Light started fading so quickly after we were freed. Thus the "Warden's Light" would be hers, the one who imprisoned Zodiark.

2.) Rather than Hydaelyn we did free ourselves with a great surge of Light at that point with slight aid from our Blessing. Again explains why it fades, but the important part here is the "Warden's Light." Because who among the Twelve has that title? Azeyma, thus meaning if the Warden comment was supposed to be about the WoL instead of Hydaelyn that gives heavy implications towards who the original owner of the WoL full soul is.

Personally I also feel if the rest of the Scions were also part of the group who summoned Hydaelyn, Twelve or not, they'd also have the Echo. But we don't even fully understand it yet though given how much we've learned this xpac that may been one of the things that will be answered in the not-so-distant future.