> 1. Members marked private being accessible from other instances of the same class [1] is counter to every other OO language I have used. What is the prior art here? Why depart from all conventions like this?

C++ works that way, and several other languages do as well. And the use case mentioned in that comment is exactly the reason: the implementation of a class knows how to access both its own private fields and those of another instance of itself, so that it can do comparisons, combinations, or other operations.