Well there is some stuff that go more indepth in the novel because the extra chapters (like how the teachers dealt with the 'evil organization' remnants, and how emmerson took the lab to himself lol), some of them got into volume extras in the adaptation, but mangasushi have never touched any volume extra ever, last volume likely gonna have an extra too, but yeah still waiting for that humans are the strongest race epilogue tl
As for Cain and Ayame, both are introduced in the prequel / Vol 4.5, yes, but the prequel didn't get translated that far, it just stopped after finishing the 2nd arc out of 3, but Cain started to become relevant around Vol.4, which is why author felt the need of making that extra volume with backstory, but explained too that since some people don't care about flashbacks, the volume can be skipped, Cain was still introduced later in the main story and then had the last volume flashback (author intended to make more .5 volumes with the past arc, depending how well Vol. 4.5 sold, otherwise it was going to stick to main story and shove them into it, but well, see how that went lol)
And yeah, there is that long strip thing, that started last year, with latest raw at 36 so far and still going, but translation seems to have stopped
Author did finish the story 'properly' in the way they know, halfassed, like every other work they have, years ago (2016ish) the story was actually completed in WN form, but author deleted it when it got serialized into LN, then in 2020 reupload like half of it somewhere else (usual thing the author do, think only Skill Taker was never deleted, and still up mostly completed)