Great post, Aspi! I found myself agreeing with pretty much everything you said.
I really like this interpretation. I was going to bring up the prologue of Hotarubi, where Bern and Lambda seem to be the same as in Umineko, but then I remembered that that prologue technically appears in an Umineko book (which is a collection of Umineko short stories). And the book is even called ‘The Last and First Gift’, implying that it’s the “end” of the Umineko “phase.”
So I’m very interested to see how and if Lambda and Bern “change” in the next When They Cry (These two are the only guaranteed appearance, thanks to their promise to meet again ‘When Something Cries’, although of course other Umineko characters may appear, too).