Not a technology. A trained Theurge of appropriate skill can sense when someone else is trying to Change a telos, and intervene reflexively to reinforce the nature of the thing being Changed and/or disrupt the opponent’s capacity to make Change. That can happen a few times in G1 Ch 4 if you’re contending magically with Theurges.
If you’re caught in the end and taken on Braveheart parade to Grand Shayard, you’ll be under constant Theurgic guard. Your hands will be shackled and muffled to keep you from cutting yourself, and there will always be an alert Theurge gauging if you’re somehow trying something anyway. Those precautions can work in the short term for the exemplary display of a captured Goete rebel…but it wouldn’t be remotely cost-effective to run a prison colony on that basis.
If I’ve mistakenly written anything inconsistent with this or opened up any plot holes, let me know. 
Well, they already kind of do. When Shayardenes call the Archon “Conte,” or talk about “a telone” and “many telones” rather than telones/telonai as the singular/plural, or a “Goete” rather than a goes, or strictly speaking, even when someone says “Karagond” rather than “Karagounid” – all of that could be called “Koinerin.” They’re mashing Koine vocab with Shayarin structure, or vice versa.
Here’s some draft text on that topic from the Cerlota dialogue:
*if wisardry = "Wisardry"
"Wisardry." She tastes it, brow furrowing. "An old Ciarreno word?"
"From stories going back long before the Codex."
"In Erezza, when not using Koine, we might say [i]magia[/i]."
"Aye, magic. They'd say that on our Coasts as well, and everyone
knows that Theurges are called magi. But up in the Rim and Westriding
we still remember more of the older Shayarin tongue. To speak of a
wisard doesn't make folk think of Theurgy. It sends their minds
further back."
Cerlota considers, then shrugs. "We also have older stories that
speak of [i]stregoneria[/i], but I would not be a strega. Those words
have too much of the echo of Goety about them. For your Wisardry,
I shall say Magia."
When the action turns to Wiendrj and Nyryal, I’ll decide the comparable terms, if any.
Nationalism, totally. Religions generally have some intrinsic tie to a compassion-ethic or at least a purportedly universal ethic, which tends to generate contradictions in the social project of using religion to oppress and exclude. Those contradictions don’t (at all!) prevent religion from being used that way, but they do mean that an oppressive social order with a religious basis has a built-in vulnerability–its critics can wield (some of) its own core values against it. It’s much easier to end up with a completely exclusionary and self-consistent nationalism.
As you can see from that answer, I’m at least as terrified by the extreme success of any source of social order as I would be by its extreme failure. The fact that a source of social order has some built-in volatility and can be used by critics of the order as well as its defenders is, to me, broadly a good thing. Anything that comes close to allowing elites complete and reliable social control – even well-meaning and righteous elites – would be anathema to me.
Radmar is much stronger. K gets by on ferocity and speed.
You can expect the general Erretsin response here to be: Avezia is too valuable for us to trade away to you on any terms whatsoever. Shayard has more than enough land. Stop being so bloody greedy, you power-drunk expansionist.
The more intentional you are about pushing it, the less likely it is that the gratitude (or the self-interest in learning the language of the koinon’s biggest and most resource-rich member) would outweigh the resentment.
No, that’s not realistic. You could train up your spies to use some weird hill dialect from the Reach, maybe, or one of the local Whendish languages even more obscure than Szert (the only one mentioned in G1), but you’re not going to be able to get a meaningful espionage boost from the language that’s already the mother tongue of a huge and well-connected population.