Both of these seem to be part of a long running series. Can you watch those shows by themselves without needing to know about the older Getter Robo and Mazinger stuff?
Reminder that Shin Mazinger Z ends in the middle of the story and it will never, ever get a sequel.
But watch it for Ashura anyway.
Oh, on the contrary, I think Ougi=Darkness is all but confirmed. Araragi didn't inform Ougi about Hachikuji, until much later.It was around 4 months after her disappearance (a few days before Hitagi End), which means it's after every other encounter with Ougi so far. I guess after the current Owari arc(or even during that), Araragi will start investigating Ougi and why he choose to reveal what happened with Hachikuji will probably be revealed too. He is aware that Ougi is "special", so he is cautious. Also, from what I understand, Hachikuji disappeared but she wasn't taken by the darkness. Ougi must know what happened, but she asked things she already knew, as usual.
The chronological order of Monogatari has quite a few missing parts, so things are certainly complicated and I may be missing something.
Yours is certainly a possible read of the situation but throughout the arc, the "Darkness" seems to have always known: the location of Hachikuji and whether she was or wasn''t acting like a proper oddity. Given such insight and knowledge, I think that by extension, it would also know that
Hachikuji passed away
This "darkness" or "law" also struck me as something much more hmmm primordial and raw and not something that would stick around for months just to mess around with Arararararagi.