Look, you can have an ambiguous ending and not cock everything up in the process.
Take
Blade Runner (the film, that is). I guess I'll spoil this in case...
Spoiler:
The Director's Cut adds some things in to raise the question of whether Deckard himself is a replicant (which was not really addressed but sort of possible in the original film cut). The ending doesn't answer this definitively and not even the people behind the movie agree (the producer and Harrison Ford think he was human, Ridley Scott thinks he was a replicant).
There are hints which suggest at the ambiguity, but the difference is that while they are mysterious, they clearly support a particular contention. Gaff is a mysterious character whose ability to predict the actions of both replicants and humans rises to an almost supernatural level. We're never told how he knows this (is he the one who programmed the false memories into replicants?), but we're given hints that he knows and that he wants to make sure other people know (Deckard's unicorn dream and Gaff's origami unicorn). It's plausible to believe he could know these things, and it supports one interpretation while still not being provable evidence that the other interpretation is false.
None of this changes the theme of the movie, which is very clear and not muddled at all (Roy Batty saving Deckard's life doesn't change meaning whether he's a replicant or not because that was the entire point).
Now you might say "yeah well none of the details change Umineko's theme either," and that's true, but the difference is whether the minor details might change something significantly or whether those minor details actually mattered for any particular reason.
It'd be like if Gaff just periodically showed up and said cryptic and wholly unrelated things that didn't help us speculate on Deckard's true nature. At the end of the film you'd be right to ask "So what was the deal with that dude in the trenchcoat?" And there wouldn't really be an answer. The origami animals and dialogue about Deckard's humanity make it clear that Gaff exists as a character and as a mystery in order to support this theme.
In contrast, you have a detail like "Why sign the message bottles 'Ushiromiya Maria?'" Thematically, it really doesn't matter what Yasu signed there; Ange didn't even believe it to begin with, so she never even suggested that Maria was actually the author. If she had used just about any other name it wouldn't make a difference. If she'd simply never signed the message bottle at all it wouldn't make a difference. Signing it Maria created a mystery for us that really ultimately didn't matter all that much. I can try to justify why it matters and interpret what that means, but it's ultimately all guesswork. It has no meaningful foundation in the work.
I don't have to justify why Gaff exists in the plot of Blade Runner, even if I can debate what the things he did meant.