After my second read through, I think I've come up with a solid theory on Elhokar's presumed assassin.
All the characters are convinced that someone is trying to kill Elhokar. The reason for this is because of the cracked gemstones in his ShardPlate. Elhokar admits to Dalinar that he cut the strap of his own saddle to force him into taking his paranoia more seriously, but does not admit to cracking the gemstones in the Plate.
"What of the cracked gemstones in your Shardplate? Did you place those too?"
"No."
"Then maybe you did uncover something," Dalinar said with a grunt. "I guess you can't be completely blamed." pp959
So Elhokar didn't break his own gems. Well, that's good. That would have been profoundly stupid of him, yes?
I think Elhokar's statement about not being responsible for the gemstones is untrue. I'm not calling him a liar, though. I just think he doesn't KNOW he cracked them.
Consider another section, where Elhokar's paranoia is highlighted.
"We go to war for years and years, never noticing the real villains, working quietly in my own camp. They watch me. Always. Waiting. I see their faces in mirrors. Symbols, twisted, inhuman." pp826
Huh. That's weird. That sounds an awful lot like what Shallan sees.
She had drawn something standing in the doorway behind the king. Two tall and willowy creatures with cloaks that split down the front and hung at the sides too stiffly, as if they were made of glass. Above the stiff, high collars, where the creatures; heads should be, each had a large, floating symbol of twisted design full of impossible angles and geometries. pp 467
...A figure that wasn't really there, a figure with a sharp, angular symbol hovering above its collar instead of a head. pp641
And five symbol-headed figures in black, too-stiff robes and cloacks. Each had a different symbol, twisted and unfamiliar to her... pp642
Jasnah tells us what they are late in the book.
"These are a type of spren, Shallan. They are related to what you do."
So, Elhokar is seeing the Soulcasting spren. Ladies and gentlemen, I'd like to put before you the theory that Elhokar is (surprise) a Soulcaster! I believe the reason behind his broken gemstones is because he is inadvertently Soulcasting and using the Stormlight and cracking the gems in the process. In Shallan's chapters, we learn that Soulcasting sometimes breaks or cracks one of the gemstones. It's not much of a stretch to think that perhaps Elhokar has accidentally broken his Plate's gems by Soulcasting.
You might ask why, then, have we seen no evidence of Soulcast materials near Elhokar? I'm afraid here I have no answers, only speculation, though I only just came up with this theory and haven't had an opportunity to go back in search of evidence. But as far as speculation goes, consider the following:
We know from Szeth's viewpoints that his Lashings do not affect Shardplate. I'm keen on the idea that the same is true for Soulcasting and Shardplate. Perhaps he is dumping the stormlight into the Plate attempting transmutation without any of the obvious effects, kind of like how a Windrunner can expel Stormlight from their bodies.
Let me know what you all think!