AY: No one said anything about turning back the clock. The OP’s question was how to kasher the army. That’s like asking how to kasher a mamzer. Neither can be kashered. And with a mamzer you can also not turn back the clock and avoid his creation. But that doesn’t negate that the answer on how to kasher him is that it is impossible.
Additionally, regarding your other comment, the Satmar Rebbe zt’l lived over 30 years after the creation of the state. And his magnum opus referenced above was indeed written afterwards in direct response to the State’s creation. So it is certainly most relevant to the discussion of the post-state situation.
Avi: R. Kasher was exposed for his notorious forgeries in support of Zionism during his lifetime and he had no response to that expose. See earlier threads for the discussion about this.
The Maharal discusses the Three Oaths in Netzach Yisrael, Ch. 24. He writes that even if the Goyim try to force us to take Eretz Yisroel for ourselves during Golus, we must allow ourselves to be killed rather than violate the Oaths. The Maharal says that these oaths represent absolute prohibitions that one must sacrifice one’s life before violating. In technical terms, these oaths are yehareg ve’al ya’avor. It is better to be martyred than to violate these oaths.