[haguruma]

Originally Posted by Renall

There is no story mechanism that would limit her selection and prevent her from finding, say, a fragmentary world in which the incident never happened because no one involved in it was ever born. Or a fragment that says "The Earth never formed, nothing happened at the time that would have been Oct. 5, 1986." There are of course reasons why she would not choose to present such a universe, but Lambda seems to state that there isn't even a choice involved, and that such fragments do not exist to be found. For that to be true, either it must not be possible for those realities to exist (patently absurd), or there must be some force actively excluding possible but irrelevant realities (there isn't).

You're applying real world mechanisms to a fictional universe that has effectively established them otherwise. What you do is like saying "It's impossible for magic X to work like this because of physical law Y" in any fantasy narrative.
Within Umineko it has been established that the multiple universes are pretty much similar to each other and only branch into minor differences...which is something that is necessary in almost every dimension and time travelling story. Of course there'd have to be an alternative where the world, Rokkenjima, humanity or the Ushiromiyas never existed in the first place...but in what way would it help to build this story?
Okay, so Bern would have had to say "There is a 0,0001% chance of this happening in the infinite number of worlds that actually include Rokkenjima in the first place. But of course there are worlds where Rokkenjima did not even exist in the first place.". Yes, but basically it could have just as well happened in Higurashi that they ended up in blank space when switching worlds...why didn't they? Because it doesn't serve the narrative.

Basically we can dismantle the whole story regarding realism and established "real world logic". But I think it doesn't benefit our understanding of the story, because what we have to do is use the established logic of Ryûkishis universe, no matter how absurd it might sound to anybody. If Bern says that she searched through all worlds and there is almost no chance of the Rokkenjima incident not occuring in any way, then this the truth of the Rokkenjima universe unless it is established otherwise.
Regarding the "the boat broke down" you could easily imagine that the radio broke down as well and nobody came to rescue them. Then the typhoon came and killed them all...end of story.