That being said, while Vlad III is indeed dangerous and difficult to deal with, the option of watching Astolfo be executed by him never existed from the start.
Shinji's goal is to seize the Holy Grail, but he can't achieve that with his power alone. Therefore, he needs allies, and Astolfo is one of the potential allies he's chosen.
What makes the best kind of ally? The answer is simple, two points:
They are unlikely to betray you.
They have no desire for the Holy Grail.
Astolfo fits these criteria perfectly. Firstly, he is a knight and follows the chivalric code. Once he trusts you, he won't betray you—the fact that he took all the blame on himself earlier is the best evidence of this.
Secondly, he has no regrets in life and no wish to entrust to the Holy Grail. He doesn't even know why he responded to the summons, often saying things like, "Ah, since I was summoned, I might as well enjoy this second life."
If it weren't for his extremely scatterbrained nature, he would be the best ally instead of just a potential one. Of course, after letting Sieg go and blocking two attacks for Astolfo, he is already Shinji's ally. From here on, as long as the circumstances are favorable, he can be fully brought on board.
But before that, they need to get through the current crisis.
As Shinji pondered how to persuade Vlad III to spare Astolfo's life for the time being, Chiron had already spoken up, not to the Grand Duke on the throne, but to the silent Caster, Avicebron.
"Caster, you previously mentioned that the homunculus might serve as the Heart of the Furnace for your Noble Phantasm?"
The Servant, whose face was always hidden behind a mask, gave a slight nod.
"Just possibly? Not definitely?"
"I cannot draw a definitive conclusion without experimenting."
Avicebron was a pure researcher, always adhering to the principle of truthfulness and never falsifying results.
Chiron continued, "So—it's also possible that the homunculus cannot serve as the 'Heart of the Furnace,' correct?"
Avicebron nodded.
"Thank you." Chiron politely turned to Vlad III and said, "Grand Duke, you heard him. It's only a possibility, not a certainty. If the homunculus cannot serve as the heart, then Rider was merely dealing with a homunculus according to his own will. If I recall correctly, we Servants have command over homunculi."
"Archer is correct," Darnic interjected.
"So, we cannot execute an important member of our faction over a mere possibility, don't you think, Grand Duke?"
"..."
"Wait—"
Before Vlad III could respond, Roche stepped forward.
"—My teacher is just being cautious. Based on the data we collected from other homunculi, that one definitely can serve as the Heart of the Furnace."
The person most displeased with Rider for letting Sieg go was undoubtedly Avicebron, followed closely by his Master, Roche. Roche held Avicebron in extremely high regard, dedicating himself to learning from and assisting him in completing the Noble Phantasm known as the "Primordial Human, Adam."
Now, all the preparations were complete, except for the "Heart of the Furnace," but the "Heart" had been let go by one of their Servants. How could Roche not be angry? After all, he was only thirteen years old and couldn't maintain Avicebron's absolute rationality—if the potential "Heart" was lost, just find another one; getting angry was meaningless and a waste of time.
Unfortunately, a naive child-like Roche was no match for Chiron, who shut him down with a single sentence.
"That's still just speculation, isn't it?"
"But, but—"
As Roche was at a loss for words, Vlad III slapped the armrest of his throne and shouted, "Enough! Archer, even if the homunculus cannot become the 'Heart,' it does not excuse Rider's rebellious actions."
Initially, the dispute over the homunculus was only between Avicebron and Astolfo, a minor conflict due to differing opinions. But once Avicebron reported the situation to Darnic, who then issued a pursuit order, the nature of the problem changed.
If Astolfo had handed over Sieg at that moment, it would have been fine. But not only did he not hand him over, he also broke through the interceptions by Gordes and Siegfried to send Sieg away. This was undoubtedly an act of rebellion.
"Rebels must be punished!"
Vlad III's words were resounding and undeniable.
"Grand Duke—"
"His Excellency the Grand Duke is right!" Shinji interrupted Chiron's upcoming argument, clapping his hands and stepping in front of him. "Rebels must be punished."
Upon hearing this, Astolfo immediately pouted, Chiron's expression grew more puzzled, and even Vlad III was somewhat surprised.
"If you agree with my view, why stop me?"
"Because there are many ways to punish, and the one the Grand Duke has chosen is the most wasteful," Shinji said, glancing back at the two with a reassuring look.
Vlad III asked, "What do you mean by that?"
"After all, this is a time of war, and it's still early in the conflict. Although we won the initial battle and gained some advantage, this advantage is far from securing victory. If we execute Rider now, it would be a loss to us and a gain to the enemy, wasting the advantage we've just obtained."
"What do you suggest then?"
"It's simple: let him die on the battlefield. Rebellion is a grave crime, and he must atone for his life, but there are many ways to die. Being impaled by the Grand Duke is one way, dying on the battlefield is another. This way, he can still create some value for our side. I suggest assigning him the most dangerous tasks in the upcoming battles, placing him wherever it's most perilous. For our side, it won't waste our combat strength, and for him, dying in the charge won't disgrace his knightly name."
Shinji's goal was to change immediate execution to a reprieve. Both Shinji and Chiron knew that they couldn't confront a tyrant like Vlad III head-on; they had to use a softer approach. Chiron had already proven that reasoning wouldn't work, so they had to try another method of appealing to Vlad's interests. As long as they could keep Astolfo alive, there would always be a chance for a turnaround.
With Chiron's wisdom, he immediately understood Shinji's point when "waste" was mentioned. He had prepared two arguments and had chosen the reasoning approach to spare Astolfo more suffering. Now that his suggestion was rejected, he quickly followed Shinji's lead in appealing to Vlad's interests.
"I agree with Assassin's suggestion. Although we have gained the Red Berserker, from Saber's description, our Lancer doesn't seem to be on our side. We must be cautious."
Mentioning the female knight who could rival Siegfried, Vlad III's anger finally subsided—that indeed was a significant hidden danger that needed attention.
"Very well, I will follow your counsel and give Rider a chance."
Hearing this, Shinji finally breathed a sigh of relief. He bowed again without showing any emotion on his face, but inside, he was sneering—though Vlad's strength had increased, his mind was not as sharp as before. Let him be complacent for now; misfortune awaits him later.
Semiramis, Amakusa Shirou, don't let me down.
P.S.: Grand Duke—why am I so unlucky? Truly, a heart-wrenching tune, where can I find a kindred spirit?