Hasn't Krishna made it quite clear that "chhal" will definitely be used in this war?
All rules will get broken in this war...
Krishna's main aim was to ensure the victory of the Pandavas through any and all means...that's it. Also, Krishna ensured that the Pandavas survived the war and beyond...
Regarding Bhishma's vow, he never took such a vow to begin with...complete ⭐️ creation.
And Karna - as I said, that was just the well-being of the Pandavas being ensured...I mean, Karna had already promised Kunti that YBNS would not get killed, and Krishna HIMSELF was with Arjun as his sarathi, so even Arjun was "safe"...
And before Kunti, it was Krishna who did the initial damage by revealing to Karna about his true parentage.