Just when I thought I was out ... you
pull me back in. 
It's a serious bit of mental gymnastics we're asking of people at this point, to forget everything we know about the novel (that's what my therapist keeps recommending) and try to intuit what the illustrations alone could spell out for a proper storyline. But I'll play along as long as I can endure it.
For those who want to follow along, these are based on the illustrations which can be found here:
https://imgur.com/r/Berserk/8hf8K/
They triumph from the beast and manage to flee from the fort. Grunbeld loses sight of his companions in the escape, and suffers from injuries as he takes a fall to evade his pursuers.
Escaping their imprisonment, while not supported anywhere, at least makes more sense than what happened in the novel -- the end of the fight was perfectly timed with a rescue operation.
He is woken by the touch of a huge wild dog who licks his wounds. Close-by, lying naked on a moose, a young girl watches him with a strange gaze. Her name is Benedikte and she is a druid, a priestess of Grant's ancient religion. She is a little eccentric, but has certain abilities, like the power to speak with animals. She provides him aid to heal his injuries, and later, through a ceremonial dance, she prophesies that he will save Grant from Tudor.
Oh, you swapped these around so that he meets Benedikte after his imprisonment, not a bad idea. This does make more sense, because Grunbeld's injuries don't have any connection, at least through the images alone.
For years, Grunbeld and his troops manage to defeat and thwart Tudor's Red Walrus Knights. But they are a small force against the might of a much larger country, and this prolonged war takes it toll on them.
I like the inevitability this scenario implies. They're valiantly defending their homeland, but it's not a defense that can last. It also plays well into what eventually happens in your scenario -- Grant either has to compromise (Edvard's plan) or go down fighting (Grunbeld's plan).
Tudor eventually proposes an accord: a right to a share of Grant's resources in exchange for putting an end to the hostilities and bestowing it protection as an ally. However Grant will also need to convert to the religion of the Holy See and end its old practices, deemed heretical. Many are amenable to this proposal, and Edvard in particular favors it. Tudor has recently put an end to its conflict with Midland, and is widely considered to have lost. They need to wash away that humiliation, and they require resources to rebuild the country. They will now focus their full might on this conflict if they can't get what they want. Grunbeld however will not yield. After all this time and all they've done, he won't come to a compromise with Tudor other than their full retreat, and can't accept an agreement that would jeopardize Benedikte's status — and life. Sigur is also tired of the war, but her loyalty to Grunbeld takes precedence over those feelings... and even over her affection for Edvard. Besides, she also grown close to Benedikte.
Wow you really put a date on this thing! (the climax of this encounter would be sometime during the 1-year gap after Guts left the Falcons). This is a really tough value judgment though, but it would offer an important introspection into Grunbeld. Which is the right choice to make? And what does his choice say about him as a person? Unfortunately, the open variables at play make this kind of a wash. There's not enough information about the people of Grant and their adherence to their particular way of life to gauge which is the right choice for the people -- whether to endure another decade or more of siege warfare, or be swallowed by a different culture. Is Grunbeld a tyrant for making that choice, or is he aligned with his people?
On its face, Grunbeld's plan to stalwartly defend until the end is fitting for him
(since he's all about defense), and utterly stupid unless there's some massive reserve army that could feasibly topple Tudor's forces in short order. All he's doing is prolonging the inevitable, and the people will suffer along the way. This all works, because though Grunbeld was remembered as a hero to his people, he's not necessarily making the right decision FOR his people. He's doing it for a woman, and for his personal grudge against Tudor. I think this would have to be hammered home in a full story in order to make it clear the value judgment he was making -- that Grunbeld is dismissing an option for peace out of hand, because of his personal feelings against Tudor.
None of that should-he-would-he talk really matters much though. Because as a future apostle, it was within causality's plan for him to grind Tudor's occupation forces beneath his giant dragon feet. Everything before that point should work as a kettle set to boil.
As the day of the next battle edges closer, Edvard comes to a fateful decision. He secretly contacts Tudor and makes plans for Grunbeld to be killed on the battlefield. He will help them capture Benedikte so they can lure Grunbeld to a specific spot, where a battery of cannons will obliterate him. Grunbeld will die a hero, true to himself, and as the new leader of Grant's army, Edvard will ensure his country can survive as a proud and autonomous nation, even if that involves some sacrifices. Then, he and Sigur might finally...
Ah, you bestowed Edvard a logical motivation for his turn. So in this scenario, he's not going "full" villain necessarily. He's removing an impediment to what he sees as his people's progress, and clearing a path to peace (and Sigur), but he can only get there by betraying his friends.
On the day of the battle, Grunbeld is as unstoppable as ever and is determined not to falter despite the overwhelming odds. He's mowing down enemies on the frontline when he spots Benedikte being held hostage. They're pointing cannons at her. He rushes in, annihilating anyone in his path, and reaches her as they fire.
I think it'd be more effective here if Grunbeld knew she had been taken hostage, resulting in him recklessly rushing to rescue her, bringing him further away from the defensive line into enemy territory, and he becomes surrounded in the process of getting to her.
He shields her with his body, and manages to save her. She's intact, but he has suffered heavy wounds. His shield is in tatters and his left arm is mangled. His right arm isn't in much better shape, and his armor is punctured. Tudor soldiers are now moving in to finish the wounded dragon. Sigur, who has witnessed the scene, tries to intervene, but Edvard stops her. She realizes his treachery and, heartbroken and outraged, turns on him. As they fight, a dying Grunbeld faces utter rage and despair. He can't be beaten! Not him! Not like this! And his call is answered. He is taken to the God Hand with Benedikte still in his arms. He is offered a choice, a second chance. And he sacrifices.
Good work remediating the nonsensical remote branding of Edvard and Sigur.
The only thing that's rocky here for me is Grunbeld's acknowledgement of Edvard's betrayal, which happens in the periphery here instead of full-frontal. The scenario would benefit from an exchange between them, but maybe that's cliché.
Meanwhile, Edvard and Sigur are still fighting. She won't listen to him. He tries to disarm her, but delivers a mortal blow without meaning to. She stumbles backward, lifeless. He falls to his knees over her dead body. This isn't what he wanted. It went all wrong. Suddenly, he looks up. A monstrous, fiery creature stands over him. An actual dragon, and one whose crystalline body can't be scratched even by cannon fire. Edvard's death comes swiftly. Tudor does not win this battle, nor does it win the war.
A much more tolerable ending for Edvard than... the novel.
A few years later, as Grunbeld lays asleep, he experiences a strange dream... A call. Someone is waiting for him in a faraway land. The Falcon.
Couldn't be
too many years later, as the Hundred Years War was already wrapped up with Midland and Tudor.
