I think this actually makes it rather doubtful wether these scenes are actually true tbh.
There’s something that kinda got sidetracked in Umineko. When we first saw Ange’s future in 1998 at the end of arc3, the scenes with Bernkastel clearly suggests that this future is “merely one probably future” with a rather low probability, but not the necessary factual future. If this is truly the future then it’s not merely a probability of a future, it’s just the future and that’s how it ends.
In arc4 meanwhile LD tells Ange that it’s not impossible for Battler to leave Rokkenjima in 1986 and come back home, but that will be for an Ange, that is, essentially, of another timeline. It will never be for “the Ange” that she is talking to.
I don’t really know what really happened there, it kinda sounds to me like something Ryuukishi discarded midway or something, but at the very least there’s nothing that really makes it certain that what we’re seeing is the future, and even if this is essentially “correct” (as in, Eva did really come back, and so the at least official events would be true) we basically know from this line of thought that every scene after the skyscrapper incident are fiction, and same for the entire scenes of Ange and Featherine in arc6. This kinda strongly suggests that these scenes are fiction as well, and I’m not sure why that wouldn’t apply to the school scenes of Ange.
It may be even more lonely to think of it that way, but as a result I see nothing that truly ascertain that Ange ever had Maria’s diary.
Everything about prime is so confusing, we really have almost no information on what could be true and what couldn’t. In the arc7 TP, Bernkastel summoned Ange and Lion to the same theater room thing to essentially torture them (let’s leave it at that), which may be me reading too much into this… but they have essentially a strong connection : They are both characters introduced by Bernkastel as only existing in fragments that have very low probability of occurring, just that Lion’s has a ridiculously lower probability of occurring.
A thing I could add is that Ange’s “death” in 1998 is part of an endgame scroll, which kinda shows that Ange’s future is part of a fiction.
I’m not really sure what to conclude there, because at the very least I want to believe that the very final scene of arc8 is actually true (that Ange became a writer) but I also can’t shake out the idea that it’s a… kinda weird thing. (Ikuko never aged, and the description seems to suggest that Tohya is now younger then the current Ange).
Ultimately tho, it comes down to this, I don’t think we have any reason to believe that Ange’s world that we see is actually anything but a fiction. I also don’t think we can be certain that the actual future is a fiction either. It’d be essentially the same as the “story arcs” vs “what really happened on Rokkenjima”. That is, saying that something actually did happen does not mean that any of the message bottles contains anything about the truth of what really happened.
And so that was a long turnaround, but I would say that it’s extremely unlikely that Ange actually had access to that diary.