Does this foreshadow any happy reunion between Guts and Casca? Or Casca being a great leader again? I don't think so.
Well, it doesn’t “foreshadow” anything as it isn’t a part of the story, but you’re still missing he big picture, as always. That statement was already relevant and has been for a long time. But Guts’ development post-Conviction Arc has been about choosing to prioritize Casca and his companions over his desire for revenge. He never abandoned his vendetta, but he is not the same person as before who would throw everything to the wayside to become a selfish monster. He has grown up, come to find things both new and old that he wishes to protect, and will in the end find a reason to fight Griffith for them and himself. His wrath and hatred aren’t going anywhere, but this is a very different story than it was twenty or so volumes ago and we arrived at that point naturally.
As for the reunion, well, as we have seen already, it was not a happy one, because of Casca’s recollection of the Eclipse. Nobody expected anything different from her. And there will definitely be a lot of difficulties for Guts and Casca. They will need to share in their trauma from the Eclipse and deal with an inevitable friction once it is discovered that their son is sharing a vessel with Griffith. But Miura obviously didn’t spend twenty+ volumes bringing them back together just to tear them apart again and bring Guts back to square one. That would be the word move Miura could make when it comes to natural plot progression and character writing, and shame on you for even insisting on the possibility to begin with.
Even with all of that in mind, it’s still irrelevant to the discussion of Casca becoming a leader.
That doesn't even begin to make sense. Who in their right mind would trust or follow a leader who they don't even know?
Farnese and Isidro would and did when following Guts. You also say that like they aren’t going to get to know Casca after her awakening. As if she is going to wake up, remember the Eclipse, scream, and then immediately head off to start leading battles against Apostles. No, obviously there is going to be enough time for her the party to get know her a little while she processes everything in her own way.
Even worse, she used to be mentally retarded.
She also used to be the commander of the Band of the Falcon and a skilled and capable warrior. Who I’ve said multiple times is the Casca we are getting back.
Something like that is literally fan-service. It's not good writing, and I don't expect Miura to take the story in that direction.
A former warrior and military commander returning to the story after a long period of absence to create a real change in the way the party operates in combat? Inverting her role from someone weak in need of protection to a strong pillar of support who holds the party up and gives them strength? All leading up to an inevitable confrontation with the man who destroyed her, with Casca herself taking charge of her fate and having agency? It sounds like amazing writing to me and exactly the kind of thing Miura would pull off.
It’s a natural progression of the dynamics of the party that is both satisfying and doesn’t compromise the progression the story has been heading since the Conviction Arc. It being satisfying doesn’t make it “fanservice.”
If anything, your half-baked, tragedy for the sake of tragedy development that not only drags the progression to a halt but tosses it all back to the starting line couldn’t even be called good fanfiction. And if you still insist on arguing about it despite so many people telling you why it is a bad development for the story, then I’m afraid that not only do you simply not understand the story of Berserk, but you also don’t have a good understanding of what makes compelling and satisfying storytelling.