The Redahn Situation
When considering whether Redahn was willing or not, I think we also have to consider the sheer timescale at hand here. I really suspect we are looking at probably somewhere between 100 - 1000 years when Miquella's plan started and when it ended. So whether or not he was willing at Point A, I believe a TON of shit happened between it and Point B.
Idk if anyone has covered this or not, so this may be old news, but my personal theory is that we are actually looking at the remnants of a failed first plan and the second plan that salvaged it.
Failed first plan: Miquella builds himself a Big Body with the intent that he and Redahn would rule together with Miquella in his new adult form.
Miquella built the Haligtree with a (or several) purpose(s) in mind. We don't know how long it took. If his cocoon was the battery that grew the tree (as we can reasonably suspect since the tree is sick now, after his removal) then he was growing for quite a long time. So long, in fact, that judging by the size of his arm in Mogh's realm, he was almost done cooking.
I think this is where it became clear, timeline-wise, that Redahn was not cooperative, whether he initially agreed or not.
In order to become a god, the Secret Rite Scroll describes a process:
"A lord will usher in a god's return, and the lord's soul will require a vessel."
I think that Miquella intended to grow himself big and when he was fully formed, Redahn was supposed to come to his cocoon and "usher in [his] return." Redahn clearly refused.
Second Plan Pivot:
We know that St Trina speaks to people in dreams. At this point in events, Miquella and Trina were still the same person.
We are intended to think Mogh is crazy when he is rambling about the dynasty they'll share, but we now know Miquella charms people to get his way. Sir Ansbach even tells us he thought his lord had been bespelled and challenged Miquella to a fight for his freedom.
If Mogh was charmed, then he is the backup plan. Miquella had to abandon the body in the cocoon since he'd never have the right god to usher him in that way. He had to pivot into the land of death/the Shadow Realm, where he could better overwhelm and kidnap Redahn after a lengthy mental weakening by scarlet rot. This couldn't have been his plan from the start, or he'd have offed himself in his kid body rather than attempt to build a grown one.
So was Redahn willing?
Honestly, I doubt he was ever willing. Notice how he doesn't appear in the pact swearing scene. We hear Miquella's side of the promise, with lengthy pauses in between, like someone else is speaking but their dialogue is gone now. Convenient, since its absence supports Miquella's narrative that this was a willing partnership.
But we know Miquella has a habit of getting rid of the inconvenient parts of people. Leda's paranoia, Sir Ansbach's loyalty to Mogh, The Hornsent's rage and quest for revenge. I don't see it beyond the realm of possibility that he would choose to simply ~not remember~ Redhan saying no.
(and how interesting that the vow scene has pauses like a voice was removed when, upon fighting Redahn, we find his voice missing.)
But regardless, no matter what Redahn said, he said it to the child version of Miquella. So, before the cocoon. Before everything. And I think the timescale here is the key. Even if he was willing when the shattering first occurred, he was clearly no longer willing by the time (possibly hundreds of years later) when Malenia came to collect him.
And to be honest, I'm not even convinced Redahn is what got resurrected in the end. That is some Pet Cemetary shit that Miquella pulled. Every other demigod speaks to us when we fight them. Redhan does not. We fight a silent puppet, more or less shaped like Redahn, but clearly made from Mogh's crucible-cursed flesh.
I think Miquella did to Redahn what Preceptor Seluvis wanted to do to Ronnie. The question of "is he willing" doesn't even come into it.
Is he even still in there? How much of what makes Redahn himself even remains?