If Imhalias was the father the baby would have been Norsirai, and I am pretty sure Akka would have remarked on that. Maybe Bakker just wanted him to be born before the NG rises.
Deep down, I do believe Akka is the baby's father, but I am having way too much fun with the "Imhailas Jr." theory not to keep trying to find more evidence in its favour.

The 6-month-long pregnancy, timeline error or qirri side effect, just lends itself to it.
Can't tell
that much of a baby's future appearance by its look at birth, anyway (Caucasian babies can appear a bit darker-skinned at birth/shortly after than they will be later on in real life, I remember that was the case with my younger brother). Or maybe the baby took after Mimara (even though most characters look like their fathers in this series...) and is paler than Achamian, while still darker-skinned than a Norsirai would be.

He definitely
had to be born before the No-God rose, and narrowly avoided being stillborn like his twin considering the short length of time between his birth and the No-God rising. He was lucky the qirri apparently has the side effect of helping with fetal lung development, among other things.

Yep, I've just done a little part of quoting these guys' statuses without conjectures.
Now is conjectures' time! :з
It's a mistake, it's a retcon, it's a different timeline (poor excuse or great plot layer, lol).
Poor Proyas, as if he didn't have enough problems, he is trying to reconcile memories from two different timelines.

I like that theory.