Shen's power can be explained by strong Memory augmented with Authority. Ella's ritual consisted of making a connection from both points. Same happens to MC - he's being pushed into dream by power with Memory lineage and guided through the process by Syla who exists entirely within memoryscape. Also Shen's ability to manifest some of his illusions physically after 4 evolution is an additional argument for his power being a Memory mix.
My theory for 10th is Lie. Truth works by imposition of concepts and events within certain physical borders, Lie destructs/ereases things and events if they stray outside of a predetermined borders like a deal or vow. Basically it's an explanation for Shopkeep's and Charlie's power.
The Narrator says that the humans who looked like Eisheth were under Shen's power. It's quite hard to explain this effect with either Authority or Memory. He would need to affect Elijah or Kira in that case.
"Eisheth's inborn instincts kick in and she leaps away, gripping every human,
now no longer under Shen's power, and lifting them into the air."
Also if it was Authority or Memory, wouldn't Shen be extremely strained by trying to affect 2 level 5s for so long? He is just a level 4.
I understand your argument, Lie and Truth are indeed opposite concepts. However, consider also that we are talking about 12 aspects of one God. The others all make sense, but Lie? Is this apostle the lie of God? Would a god lie? In this light, even Illusion is a weak option. Probably the most sensible options would be Imagination and then Dream.
One important thing to note, since a lot of us are going by the apostolic statements ("I'm Gods X"), Truth is the name of the Apostle.
So the statement would be "I'm Gods X, something something, the something Truth".
X could be reality, actualization or actuality, domain, etc.
The 10th Apostle could be God's imagination, possibility, innovation.
The reason I think Lie wouldn't work is because it feels like a colloquial term with a negative load. Illusion works better.
The important think to note here is that the terms have to be tied to a overarching concept. Truth and Illusion work greatly together to affect Perception.
Both True and False things affect our perception. As long as they feel real to us, then it doesn't matter if they actually are or not. It's how placebos work. It's how praise works. Basically a lot of things can be attributed to it.
The "feeling of home" can be an illusion for example. Perhaps at first, the particular house had the emotional load that made us feel like we are at home. With time, the things that made it this way may change so they aren't true anymore, but the feeling persists.
Doing things to forget your worries can be an illusion too. You had a great night with your friends out, you felt better, but reality hits your hard the next day. It wasn't necessarily the fact that you drank or had a stimulating conversation that made your happy. It was that for a while, your forgot your troubles. You forgot reality.