Hmm I think we got tons of answers by now! Kumeta-sensei pointed out those "7 mysteries" (mentioned in guidebook to celebrate 20th volume) and now all of those milestones she pointed out got their answers! There are still many to go, but we have a lot of established facts and identities.
And if you read carefully, there is an answer in this very chapter "how" to achieve their goal, it's hidden in Nishina's words
Really? All I took from what nishina said, was that he wanted them to kill him and the spirits while they possessed him, using the forbidden spell. And that was
explicitly explained later by veronicas group (adding to kill veronica and zeze that way too, for the spirits they carry), so can't be what you refer to being hidden and having to be carefulyl read to notice.
So nothing about how to achieve their goal (fix their lifespans, and ensure the memories and magic doesn't mess up their new lives or otherwise overly affect the new world) from what I could see, all I saw was that idea, going entirely contrary to their goal.
Aside from that, there's also the fact of how absolutely all of that were pure speculations: the spirits motives, the church's motives, that there would be an issue to leave spirits be, that they can be destroyed, that all of them are truly in these 3 and not everywhere in the water, leafs and rocks and overall allows the usage of magic for all who can cast magic, and so on. Overall they have really been making quite huge leaps of 'logic' recently in the recent developments, but this chapter's was
really without any basis.