My wild guess is that David is Lucia's father. It's only a hunch based on (reading too deep into) David's reaction to discovering Hunter and Drew were fucking Nora and Keira. So take it with a grain of salt.
I had the same thought in the past. But, after reading the David's journal part again and again, i think this is not the case. When Keira got pregnant, David and Ophelia (+Aliza) lived on their own for months. Yeah, David's reaction was extreme but it was understandable. Hunter was like a father to him and Drew like a brother. His whole world collapsed when he found out what is going on in the family for YEARS.
Now that i think about it. I was so sure, that Drew and Hunter are "always" lying. BUT! What if everybody is telling the truth? Maybe the dad of Lucia is really just a random guy. After Lucia was born (or during the pregnancy), Drew take interest in Keira. Hunter noticed it, and started to train Drew, but he was loyal to Annabelle. After Annabelle died he joined to the party. And we know what happened next ... David became aware of the situation and chase Hunter and Drew away.
But, why did Drew agree to do the paternity test if Lucia's father is some random guy?
- First ... To bring Keira (and maybe Nora, the grandmother) back. They can be helpful tools in Drew's hands, especially if they have some kind of loyality after all these years. Why would they be still loyal? They had a relationship for years, so maybe it would be like a conditioned reflex.
- Second ... It is a good distraction. The MC is convinced that Drew or Hunter is Lucia's father. But, he thinks this, because David wrote it in his journal, but he could be wrong. Still, it would help the MC to build a case against them. Good plan! On the other hand, if this guess turns out to be false, it would basically be just a huge waste of time.
This whole thing is just an assumption from me. But i wouldn't rule out the possibility, that the MC is getting bamboozeled by Drew with this paternity test. I mean she and Hunter are not stupid. They definitely know the MC is trying to do something.
But, we will know more, when Keira comes back (in the next chapter?). I am the most curious about the affair between Drew and David. I don't know how a relationship could form between them, after everything that happened. Is there a chance that Drew looked so different after the surgeries, that David didn't recognize her? So, basically did David think he had an affair with a random futa?