Wow, there's a lot to comment on in this thread.
I think the primary connection between Seons, spren, and Nightblood is how they get their Cognitive aspects. We don't know a lot of Realmatic theory, but it seems to me Syl is taking some of Kaladin's Cognitive power, and she's trading it and giving back some Spiritual power in the form of Surgebinding. Seons could be drawing the Cognitive aspect from their master as well.
The problem with that way of thinking about it is that Nightblood doesn't bond with just one person. In fact, there's no indication he draws his consciousness from any source. He got it all in the front end, with the thousand Breaths. That would be enough Spiritual power that, with a Command (which would act as a sort of Cognitive seed), can bring Nightblood to sentience. It's interesting that Brandon calls Nightblood "almost-sentient." I'd imagine that means he is "pretty close to sentience but still is an imitation of it."
So okay, let's skip the whole issue of where they get their Cognitive power, because the mechanics of that seems to differ between books. Syl's consciousness, I think, comes from the Nahel bond. Nightblood's comes from the Breaths plus a Command. Seons come through mysterious processes that we don't yet understand.
I would note that the one indication of Splintering that we have doesn't deal with the consciousness of a Shardholder, merely the Shard's Spiritual power. The power is what Odium Splintered from Aona. So I don't think it is accurate to say that Seons are "aspects of Aona's consciousness." That consciousness died. Its the power that is being split, and Seons could be those Splinters of Aona.
(Part of the problem is that I would love to say "They are Splinters of Endowment" or some Shard name, but we don't know Aona's Shard, so we're using some imprecise language)
Of course, they aren't just Splinters of Aona. Splinters are like Divine Breath, they are not sentient in their own right. There needs to be something else to give them that Cognitive power. This could be how Seons are "created." You grab those Splinters, and then give them an Aon to focus their consciousness. Or something.
Now, Seons are certainly of Aona, so their Devotion/service aspect makes sense. They are Splinters of that Shard, and that Shard had a specific Intent which colors the way Seons act. That is very similar to the way Syl operates. I'd need to read Way of Kings again to see if Syl does, in fact, act like some fragment of Honor or something.
However, that also has a problem, because it seems Honorspren aren't necessarily Splinters of Honor. That largely depends on when Honor died, so they could be Splinters, but we also need to be open to the idea that they aren't. The only Splinter we know of are the Divine Breath. Nightblood is a lot of interesting things, but I don't think those thousand Breaths he is Invested with makes him a Splinter. He's a different thing.
Brandon's comments could simply refer to how things gain sentience rather than any information about Splinters.
About the Reod thing -- I’m with CrazyRioter on this one. I could have sworn that Brandon said the Reod was just an earthquake. I guess you could blame it on Odium on killing the Shardholders, but that’s like saying that everything can be blamed on Adonalsium Shattering. Technically, it’s true, though it isn’t immensely illuminative.