Having some means (as these C++ proposals all do) to explicitly say "I understand that you can't see why this is correct but it assure you it is" would be fine,

Having some means (as these C++ proposals all do) to explicitly say "I understand that you can't see why this is correct but it assure you it is" would be fine, and needn't be introduced to beginners at all. The problem as usual in C++ is that All The Defaults Are Wrong and because they're defaults we need to warn beginners about them. You won't write Rust's MaybeUninit<T>::assume_init() in your first program by mistake, whereas the equivalent mistake in C++ happens easily because it's the default.