Maybe I am reading into it too optimistically, but one thing that is probably true is that Mori knows the (very) broad strokes leading to the ending. He doesn't know how they were supposed to be executed, nor the people at studio Gaga know how to do it from scratch, but at least one "big thing" before the final confrontation between Guts and Griffith needs to happen, and he plans on delivering it somehow.
In principle there is no contradiction between the fact that the volume that just came out is completely uneventful and the fact that this section of the story, in the planning phase, would have been very hard to construct. The poor quality and uneventfulness is a Mori+studio Gaga issue, but the fact that constructing the lead up to this extreme future event was difficult is believable.
What Mori is saying is really not that mysterious, he's just referring to the fact Guts is down in the dumps. He's hyping it up. And he doesn't say it was hard to construct, just that Miura was "worried" and "suffered" when he was devising it, "like the Eclipse".
Except if we look at what Miura has actually said about it over the years, it wasn't an agonizing decision for him to kill the Band of the Falcon or anything. It had been planned from the start (otherwise there'd be no Black Swordsman) and he went as far as to say he did it "with great serenity" in
this Italian interview from 1996. No doubt the Eclipse demanded a lot of work to actually draw, of course, but that doesn't seem to be what Mori is talking about here.
Furthermore, in his last interview Miura said he had decided on the story "up to the latter half of the Elf Island chapter", and that he would be focusing on drawing that from now on. He knew what the main beats would be until the end, but it wasn't solidified to the point of having defined specific scenes or written dialogue, as far as we know. So that doesn't fit with Mori's account. Miura also said separately that while the story would move beyond the "journey" format, Guts would remain the main character, which I don't think is the case in volume 43.
So, my optimistic takeaway is that the next volume should contain a bastardized version of a big change/event that Miura had planned as a necessary step towards the ending. Maybe afterwards someone will be able to deduce how Miura would have done it?
You know, I'm all for giving people the benefit of the doubt, but after what we got in volumes 42 and 43, I really have no optimism left for what's to come. The believable "broad strokes" of volume 43 come down to "Guts' group meets with Silat's group" and "they fight Rakshas", neither of which required any insider info to figure out. And they were drowned in a sea of nonsensical stuff that basically retconned half the story.
