Tettidesa

Parts of this is very possible, but do bear in mind that the Sibling was a very purposeful creation of Honor and Cultivation, and wasn't really something that just followed from their investing in the planet/system. Therefore I don't think we have reason to believe that there will be a naturally occuring Honor/Odium and Cultivation/Odium bondsmith-spren. I guess it's possible, but we dont really know that it is something that is bound to happen.

This is incorrect. Honor died shortly after the Recreance, which definitely happened after The False Desolation. I guess it's possible that there was a large spren similar to the proto-Nightwatcher and proto-Stormfather that was subsequently infused with one or several of the Rosharan Shard's essences, before being made into an Unmade. This idea makes some sense, and I believe it's somewhat mentioned in the newest Shardcast. Unless there's something im missing though, there isn't too much textual evidence for it as of yet. As for the other Unmade having any relation to Shards other than Odium, there is an AMA from a few years back, where Brandon explicitly states that these Splinters of Odium. While this doesn't nessescarily mean that they aren't also infused with the investiture of other Shards, it does seem unlikely. Is it even possible to corrupt non-spren in the same way that was attempted with the Sibling? I do kinda like the idea overall though, Sja-Anat being something infused with Cultivation investiture hence the enlightening of spren, maybe Nergaoul being somehow related to the new Valor shard. I don't think this would be the case with all the Unmade though.

They probably did, but i doubt there's anything meaningful that occurred in that short time span, we saw with Ishar that it only really lasts for a minute at most.

A growing void? Clearly this is the ever oxymoronic Black Hole Light.

Correct me if I'm wrong, dont have my copy handy right now, but doesn't Rayse call him Cephandrius in that scene? Surely that implies that this is the name Rayse knew him under originally. Obviously that doesn't need to mean that Cephandrius is his first, real, name, but surely it is somewhat implied. Cephandrius is also the name Leras/Preservation uses, so it's probably his real name. This is somewhat in conflict with this WoB where Brandon says Cephandrius is just a very old alias, and not his real name. Not sure if this has been retconned, or Hoid had already been around for a while prior to meeting the original Vessels.