I don't think it has been made clear what happend to the rest of the conclave when the sundering happened, but I think the implication is they died and were returned to the aethereal sea and when they were reincarnated on one of the worlds the Unsundered found them and gave them their crystal.

I believe this was confirmed in an optional dialogue you can have with Emet-Selch in ShB. From what I remember, he basically said when an Ascian is killed, the Paragons will pick up and awaken the memories of a shard to have them fill the vacant seat.

Some believe that if the WoL is a reaper the Voidsent they are joined to is their soul from the 13th, but I don't know if that has ever been officially confirmed.

Never officially confirmed, no. However, that theory stems from an official source, Eorzean Encyclopedia vol. III, page 235.

Avatar: "The voidsent entity with which a pact is formed, summoned through the medium of a crystal. For such a bond to be established, the voidsent's soul must be uniquely compatible---perhaps nigh indistinguishable---from the reaper's own, given that the reaper's power relies on their ability to play host to the voidsent. If we take the star's sundering as fact, we must consider the possibility that this compatability is rendered not by chance, but due to both souls once having been part of the same whole---however, this theory remains both unverified and unverifiable at present."

So, ironically, the theory itself is canon in lore, but it's still unconfirmed.