Vasili, Oleg’s older brother, arrived in Czechoslovakia and joined the local KGB network of illegals*, whose mission was to saboteurage the Czechoslovakian “counter-revolution”.

"saboteur" is a noun (a person), not a verb

The KGB had thought outdevised a propaganda campaign to give the false impression that the country was on the brink of a massive, bloody uprising fuelled** by the West.

What you wrote is correct, but "devised" sounds more precise since it requires careful planning.

The KGB never applied any ethical self-restraint on itself, and so it, and ruthlessly exploited any brweak to the fullness.

"to the full" is redundant, since "ruthlessly exploited" already conveys this. I feel "break" is the wrong word.

The constrast between the two brothers was plain, yet concealed, for they always kept their feelings from one another.

While Vasili was at the very forefront of the KGB counter-operation, Oleg was outwardly adamant, but in reality only hoped, that the USSR wouldn’t dare to intervene militarily.

In fact, the grim sight of the construction of the Berlin Wall had haunted him ever*** since.

Here, it's note clear what "since" is referring to. Since when? Since its construction? Since the invasion of Czechoslovakia?

Yet he had never come to harbour negative feelings towards the KGB, unlike for the Soviet Union.

But now hie feels alienation from the KGb turneded, and starts to absolutely loathinge the KGB.

First, the "turned to" should be "turned into". But it would still be incorrect since "alienation" is *from* the KGB ("the KGB alienated Oleg"), whereas "loathing" is *towards* the KGB ("Oleg loathes the KGB"): the directions are different. Moreover, the story is yet to establish that Oleg was alienated from the KGB, but you're using it like a premise, as if the reader already knows this.

Anyway, nNot long after hearing of the invasion, he called his wife to share his outrage.

"Anyway" sets a light-hearted tone, and isn't well-suited to the context. You might use this when talking on the phone and want move onto another topic. Moreover, the use of "Anyway" makes it sound like you're changing topic, but you're not: it's still about his outrage.

He knewwas aware that the Danes had bugged his phone.

Correct, but it's disconnected from the rest of the story.

In fact, itSo calling his wife was a veiled attempt from him to hint he was not an unquestioning Soviet and was up forwilling to engage in “discussions”.

*Illegals are, in intelligence parlance, spies who blend into a target country posing as ordinary citizens, unlike those who work under diplomatic covers.

By “fuelled” I meant that the West was behind it.

Yes, you used it appropriately.

That the West backed the rebellions against Communism, issuing guns and money as well as spreading anti-communist propaganda.

This is not a sentence in itself, and should be merged into the previous sentence. Consider using "resistance" instead of "rebellions".

***Can I remove "ever"?

In both cases ("ever since" and "since"), it's not clear "since [when]". If you add "ever" it emphasizes how it's a long period of time. From a grammar perspective "ever" can be deleted, but it may be an important detail.

Summary noum.

Vasili, Oleg’s older brother, arrived in Czechoslovakia and joined the local KGB network of illegals*, whose mission was to saboteurage the Czechoslovakian “counter-revolution”.

The KGB never applied any ethical restraint on itself, and so it ruthlessly exploited any brweak to theness fully.

While Vasili was at the very forefronthead of the KGB counter-operation, Oleg was adamant, in realityor maybe just hoped, that the USSR wouldn’t dare to intervene militarily.

But now, his alienation from the KGbB turned to absolute loathing.

*Illegals are, in intelligence parlance, spies who blend into a target country by posing as ordinary citizens, unlike those who work under diplomatic covers.

By “fuelled”, I meant that the West was behind it.

***Can I remove "ever"?

Maybe in some spoken dialects but it feels right to me here.

Vasili, Oleg’s older brother, arrived in Czechoslovakia and joined the local KGB network of illegals*, whose mission was to saboteurage the Czechoslovakian “counter-revolution”.

"saboteur" is a noun only. Saboteurs engage in sabotage, saboteurs sabotage things.

The KGB never applied any ethical restraint on itself, and so it ruthlessly exploited any break to the full.

Unclear what "break" means here. Weakness? Opportunity?

The constrast between the two brothers was plain, yet concealed, for they always kept their feelings from one another.

While Vasili was at the very forefront of the KGB counter-operation, Oleg was adamant, in realitor at least desperately hoped, that the USSR wouldn’t dare to intervene militarily.

"in reality" didn't quite seem to fit.

But now his alienation from the KGbB turned to absolute loathing.

*Illegals are, in intelligence parlance, spies who blend into a target country posing as ordinary citizens, unlike those who work under diplomatic covers.

Or something like "blend in among", or "who go to a target country and blend in".

By “fuelled” I meant that the West was behind it.

A natural expression and it was very clear what you meant :)

***Can I remove "ever"?

You can, but it's not as natural, I think it would be a kind of writerly, poetic style. Conversationally "ever since" is super natural here and very few would omit "ever".