Was ~ざりけり used in place of 〜けらず, for example in 知りけらず, to indicate the past tense in the negative form? Did 知らざりけり exist?
〜ざりけり certainly existed and was very widely used. This definition includes a couple of examples:
I believe that 〜けらず was actually far less common than 〜ざりけり, at least from the Heian period on, and the entry for けり in 大辞林 seems to confirm this – in fact, it suggests that 〜けらず disappeared entirely. (The main definition indicates that けり has no 未然形, so there would be nothing to attach 〜ず to; a note at the end of the definition clarifies that 未然形「けら」は、「けらず」「けらく」の形で上代にのみ用いられる and cites an example from 万葉集.) Also note that 〜ざりけり can and often does have an exclamatory nuance. I'm not sure 〜けらず, on its own, conveys quite the same feeling.