Except the "Third Domain" inhabitants, which are in-between, are only witches too (Piece and Self Insert Feath) Therefore it makes sense that it includes both.
Featherine isn't exactly a good example to use, since she is narratively treated as an entity of abnormal standing and described as above Witches in general in spite of technically being considered a Witch herself, which, going by the stated nature of her Study, applies to her avatar form too. So she'd be above the "middle" that Lambdadelta describes nonetheless.
Piece directly serves under her much like Ange and Bernkastel did (Both obviously being far from surpassing the World of Witches), so she'd just be an entity of the same vein as them: Someone from a lower realm who is allowed to exist within a higher one by the grace of an entity presiding over the latter. (Been a while since I've read Last Note of the Golden Witch, though, so you can correct me on that one if necessary)
It wouldn't really change Lambdadelta knowing about Feath whole thing though.
It would, since there isn't really much to suggest those are Lambdadelta's thoughts in the Visual Novel. The novel itself has a character's inner monologue hijacking the narration in multiple instances, so if that description was intended to be her exact thoughts, it'd just be explicitly written in the first person and from her viewpoint.
The manga changed that a bit mostly because of the absence of a third-person omniscient narrator, and in that continuity, there is not only no mention of Featherine's Avatar being in a realm higher than the world of the witches, but also direct statements establishing her to live within it. So, yeah, while the manga does favor your interpretation, as I said, it's pretty clear that the differences between it and the VN can't really be reconciled in that regard, so mixing up the two doesn't work as an argument.
Well, as shown before, the kanji used is more of a "stepping in" like you would use it in real life.
With the "she'll becomes like you" thing, it is more like how you would say in real life that someone started to look as fit as this other guy.
God I suck at explaining correct meaning
I don't exactly see a difference, though. Bernkastel still says that Beatrice already surpassed Witches as a whole and is continuing to evolve towards Featherine's level. That part is what's relevant here. Unless the "She's already beyond Witches" statement is absent from the original, my pont still stands.
It considers that "no endpoint" means there's litteraly no end and that's it refers to the "Third Domain"... somehow (despite the realm not being a thing at that point in the story), meaning it has infinite steps you can never reach to the end after the witch domain.
And the very fact that Featherine exists obviously makes this interpretation impossible. In addition to the TIPS clearly referring to Voyagers like Lambda and Bern.
You consider Featherine to have once been a Voyager yourself, though, based on a few of your comments in the posts above, and also still consider the stairway leading to the Creator to be one of infinitely-many steps (Thus making a realm beyond it unreachable by a gradual ascent anyway), so your proposals don't really fix those so-called issues. Granted, I think I've seen another translation of that description of the Voyagers' journey using the term "destination" instead of "endpoint," so, might as well get the Japanese version of that scan, no? It might clear up some doubts.
Anyway, my thoughts on a possible counter-argument that could still be made even under my view were more-or-less this: Given that the journey of the Voyagers as a whole is what is stated to have no endpoint, it's possible to argue that Lambda was just mistaken about the world of witches actually being an infinite ladder, thus meaning the only hierarchy that's actually infinite is the third one. I think Darksmash proposed something similar up there, even? (Basically just skimmed through the relevant responses after reading through the OP, so, sorry if I missed anything)
Although the thing would be that while the concept is in the witch Domain (since lambda & coe are in it); it doesnt make sense for it to be above all of it when the true form of the witches are far from the end
Being far from surpassing the ladder only applies to the personification of the concept, and not to the concept itself. Given that the latter can still exist just fine without the former (Seeing as Battler was still affected by it even after Featherine killed Lambda), arguing that the law of certainty just expands alongside her (Or something similar) seems a little far-fetched. Lambdadelta herself is very clearly secondary to its existence, and the fact she's, well, just an
embodiment should make that obvious.