The enmity between Black Manta and Aquaman takes a giant twist on the final page of this issue.

Supposedly, Aquaman killed Manta's father.

My question: Is that intended as a replacement tragedy in their relationship, overriding the death of Aquababy, or is it an additional tragedy, giving Manta the motivation for attacking Arthur's family. Did Aquababy's death happen now, or not?

it probably still happened but now manta just had a reason to go after aquaman

I doubt that Aquaman had a kid in this universe