So basically I'm trying to figure out how Sapient Cognitive Entities are created, what they are, and how they work.
Spren and Seons are the most obvious examples. But seem to be fundamentally different on several layers. Spren are said to be manifestations of Cultivation's Investiture (might be mostly speculation, but irrelevant either way), but especially higher spren seem to be of Honor. Surgebinding is Honor's Investiture, and uses the bonding of higher spren to manifest. More sapient spren seem to refer to Honor as their father. Indeed there are direct references to Honor creating highspren. These are sapient when they exist primarily in the Cognitive Realm, but they require a human bond to maintain sapience in the Physical. Perhaps if they are pieces of Honor (Splinters), or at the very least collections of his Investiture, they don't have enough Investiture to exist simultaneously in all three realms. So, when dragged into the physical realm, their essence is stretched so thin they can't maintain a sapient mind. They need to bond a human to regain this by sharing the human's Investiture.
There are a few theories i have as to why this is. My favorite of which is this: they are Splinters of Honor. They only have enough Investiture to exist in two realms at a time. In their natural state, I believe that would be the Cognitive and Spiritual Realms. When first forming the bond, and transferring to the physical realm, they have to separate from the Spiritual Realm. Tear themselves from it, really. I think this is why Pattern describes it as an "extremely unpleasant" experience, why they lose most of their memories, and all of their sapience until their bond strengthens. To me this means that self-aware intelligence is not a result of presence in the Cognitive Realm, but presence in the Spiritual Realm. [All objects and creatures exist in all three realms, but that is different from having a presence and Investiture there. A rock had a soul and cognitive presence, and can see itself, even "think" to a small degree. But that is not the same as sapience. A rock also has no Investiture. Humans are all born with a presence of Investiture, the "spark of life"] The other possibility is that since spren exist in the Cognitive Realm, the more people thought of them as alive and self-aware, the more they saw themselves that way. And due to the nature of the Cognitive Realm, and the power of human thought in that realm, they became that which they were perceived as. This theory has a number of problems. Especially since spren existed on Roshar before men did, although we do not know whether or not they were self-aware then. We do know that spren only started bonding with people after humans showed up, but that may just be a limitation of the Listeners' anatomy. An explanation could be that spren are of Cultivation, but men are of Honor (Thus, "Child of Honor"). So while the spren were originally not of Honor, the bond with the more solid creatures Investiture-wise makes Surgebinding of Honor. This also had problems, even before it was revealed that men were originally of Odium, not Honor. So the only answer seems to be that highspren have their sapience independent from men, and are splinters of Honor (confirmed on multiple accounts). But since we know that Honor created many highspren, this tells us a lot about how Splinters are formed if they can be made voluntarily by the Shard.
For this let's move onto Seons. Seons are Splinters of Devotion, but his Splintering was caused by his death. TheSeons were created after the Splintering of Devotion, by people with access to the leftover Investiture. Seons are also self-aware and can form bonds like spren, although without the same results. When a person with a Seon is taken by the Shaod, their Connection is severed, and they float around mindlessly, extremely similar to that of a dead (broken bonds) spren. When a Seon's human dies, it immediately searches another person to bond with. I don't think Seons are capable of retreating into the Cognitive Realm like spren can when their human dies, so I believe if a Seon fails to find a bond, they will find themselves spread too thin and behave similarly to a spren at the beginning of a bond. They are fundamentally similar on several levels, and I believe they are both Splinters, but they were created in different ways. (This also raises questions as to what differentiates a person from a Splinter. If I had to guess I'd guess that Splinters only have presence in two realms naturally, humans have presence innately in all three).
Now let's move on to another planet: Nalthis. Because there is another sapient cognitive entity that offers something very interesting. Nightblood, and others like it. They are sapient, can form bonds, and need extra Investiture from humans (or any source really). The only difference is how much more Investiture Nightblood needs, how much faster it drains, and the fact that its "grounding" Investiture doesn't come from a Nahel bond. My theory is that Nightblood is a Splinter of Endowment. He was created by gathering a lot of Endowment's Investiture in one place. WOB says that large amounts of Investiture are going to pierce the realms. So much Investiture concentrated in the metal pierces all three realms. This gives the sword a presence in the Spiritual Realm, and thus sapience. I think this is why it drains Investiture so much faster, the Splinter was created artificially, and is unable to sever itself from the Physical Realm due to the nature of its creation. So it needs that extra Investiture to sustain itself in all three realms instead of just the Spiritual or Physical. This could also be because artificial creation was done by someone without a good enough grasp on Realmatic theory that didn't know what they were doing, resulting in Nightblood being more "porous" to Investiture. It could also be that Nightblood was made without solid enough Intent and tries to gain as much Investiture as possible in all three realms without limiting itself. Also still, it could be that due to the nature of Endowment's Investiture, it can be easily given, taken, and stored. There are tons of reasons that could explain why Nightblood drains Investiture faster than other Splinters, but whatever they may be I still believe him to be a Splinter of Endowment.
We know a Shardblade is a Splinter manifesting in the Physical Realm. In Oathbringer, Nightblood and Vivenna's blade aren't described as "like Shardblades", they're described as Shardblades, even by Vivenna. She does go on to call it a cheaper imitation of other Shardblades, and regards other Shardblades as superior, but she never retracts or contradicts her statement that it is a Shardblade.
[Also totally not relevant but Nightblood's aluminum sheath doesn't keep Investiture from coming out, it keeps it from going in. Which is just interesting because that makes Nightblood always endothermic, which breaks the laws of thermodynamics like Sanderson said many of his magic systems do.]
Tl;dr: I think Splinters like spren and Seons can only naturally have presence (maybe Investiture) in two realms, and the require a bond to sustain a presence in all three realms by using some of the human's Investiture. I also believe that presence in the Spiritual Realm is what grants a creature sapience.
Here is your official RAFO. But it comes with a, "You're thinking along the right lines" to accompany it.