Szeth as a cognitive shadow.

[FatherV]

So, with the latest chapter, Vasher gives us his own theory of cognitive shadows and his own revelation that he most likely is one, based on the pattern of behavior of Endowment on his own Shardworld (all Returned die and then are without memories). In enunciating his own theory of the genesis of cognitive shadows he says:

Kaladin hesitated. “You… died?”

Zahel nodded. “Happened to your friend too. Up in the prison? The one with… that sword.”

“Szeth. Not my friend.”

“The Heralds too,” Zahel said. “When they died, they left an imprint behind. Power that remembered being them. You see, the power wants to be alive.” He gestured with his chin toward Syl, flying down beneath them as a ribbon of light. “She’s what I now call a Type One Invested entity. I decided that had to be the proper way to refer to them. Power that came alive on its own.”

“You can see her!” Kaladin said.

“See? No. Sense?” Zahel shrugged. “Cut off a bit of divinity and leave it alone. Eventually it comes alive. And if you let a man die with too Invested a soul—or Invest him right as he’s dying—he’ll leave behind a shadow you can nail back onto a body. His own, if you’re feeling charitable. Once done, you have this.” Zahel waved to himself. “Type Two Invested entity. Dead man walking.”

According to Vasher, either a soul dies with too Invested a soul or you can invest it right as he’s dying.

With Szeth, he was falling because he rejected the Honorblade, gave up all Stormlight, and died. In chapter 88 of Words of Radiance, we have this:

“I waited until you crashed to the ground,” the man said, “until you were broken and mangled, your soul cut through, dead for certain. Then, I restored you.” 
“Impossible.”
“Not if it is done before the brain dies. Like a drowned man restored to life with the proper ministrations, you could be restored with the right Surgebinding. If I had waited seconds longer, of course, it would have been too late. But surely you know this. Two of the Blades held by your people allow Regrowth. I suspect you have already seen the newly dead restored to life.” (Quoted from the Kindle version)

He doesn’t say that’s what he actually did though. It’s “could”, “two blades...allow.” Of the two options in Vasher’s scheme it can’t be like Kelsier an intent to remain while overly Invested, so it must be the other of being Invested “as he is dying.”

in Oathbringer chapter 90 we get a limited point of view, but its source is only the whisperings Szeth hears from other Skybreakers. 

“The Herald had used a type of fabrial to heal his body before his spirit departed. It had almost taken too long, however. His spirit hadn’t properly reattached to his body.”

“...The other five kept their distance from him. Perhaps it was because of the way he left a glowing afterimage when he moved: a sign of his soul’s improper reattachment.” It is not stated, but the source is likely these same whisperings.  Some have attributed the reattachment to a healing fabrial, but it need not be so. It doesn’t even have to be investiture native to Roshar. As we saw, Nale didn’t confirm a darn thing about how he did what he did, nor that what seems to be unusual to the Skybreakers isn’t exactly what is supposed to happen with the Investiture he actually used. Nale’s hidden things from them before.

This assumes that Vasher’s theory is both accurate and exhaustive, but given the actual evidence, Nale could have used a Dawnshard for all we can verify. While an otherwise unheard of but theoretically more plausible healing fabrial may be the instrument of Nale’s action here, we don’t actually have any firsthand evidence of how Nale did it, only the effects themselves.

 

[Halyo_Alex]

Personally I think if a Dawnshard healed Szeth, then it's a pretty cremmy Dawnshard. If they're that powerful, I wouldn't expect his soul to be "improperly reattached"... But a fabrial? Perhaps experimental in design? Yeah. That would make a lot more sense.

[Quantus]

I dunno, I definitely read it as Nale confirming it was a ReGRowth effect, one that only worked because he was healed in the window before his brain died.  If he were using a Dawnshard, I think it would imply that a Dawnshard can simulate traditional surges, rather than actually requiring a Dawnshard to accomplish what happened to Szeth.  Vasher did go on to describe a lesser version that more of a one-time second chance, rather than the immortality of Returned or Heralds. 

Quote

 

or the weaker ones, just kill the body again, make sure no one Invests the soul with more strength, and they’ll slip away in a few minutes. For stronger ones… well, you might be able to starve them. A lot of Type Twos feed on power. Keeps them going.

 

I think that's Szeth: an emergency injection of Regrowth was able to get him past the grey area between body death and brain death.  His soul had already started to detach and move on causing the odd spirit detachment (taken right out of the Riddick franchise :)).  For practical purposes I think he will grow and age and eventually die at all the normal rates, with no need for an Investiture source the way the Returned are confirmed to need, and Heralds and Fused are assumed to by Vasher. Past that it starts getting into the more philosophic and intentionally left vague (by WOB) question of whether the Spiritweb and Soul are different things, and whether Investure actually stalls the travel to Beyond vs leaving an Investiture shadow.  But that's asking "What is a Soul?" before you get to asking "Does the Spiritweb Qualify."  Vasher has definitely formed an opinion, one informed by his long tragic life as a Returned.  But his is just on POV on this, which one of the Big Questions in the cosmere

 

PS:  Using the Quote tag function will make your post a lot easier to read.