I like to think of the witches representing ideas, with the “higher” witches being ideas of higher abstraction. We know in canon that Beato is just an idea— the idea that the message bottles and forgeries are unsolvable. Yasu projects herself onto this Beato character because she too, in a sense, is unsolvable, but Beato’s core function in the story is as a concept.
I don’t know how well the idea of witches as allegory extends because it’s been a while since I’ve read Umineko, but it explains Bernkastel and Lambdadelta really well. Bernkastel obviously represents the notion of “miracles” and Lambda is the notion of “certainty”. It reveals a really interesting dynamic between the two WTC entries:
(spoiler for Higurashi)
[spoiler]In Higurashi, the central conflict is Rika’s struggle against a certain fate. The “miracle” is tasked with overcoming “certainty.”
Umineko is the exact opposite case. The central thematic conflict involves Ange being unable to accept that her family is actually dead (as explained by EP8). The entire series, she simply refuses to leave the matter behind her to move on with her life. She holds on for an explanation which will never come, an explanation which lets her family live. The villain is Bernkastel, and Bernkastel is manipulating Ange since the beginning. Umineko is a case of the “certainty” needing to overcoming the “miracle.”[/spoiler]
Higurashi and Umineko actually produce completely opposite messages, which I find pretty interesting.