Why didn't Varus fuse with his son?

The original story of the "Varus" character was pretty closed in terms of story, as it wasn't really possible for the character to do much other than succeed or fail at getting revenge and then dying. Nothing else would make much sense. So it was a good idea to rework him to be possessed by a Darkin. I know some people might not like that change, but I think it's for the better.

But then, the idea of a person possessed by a demon is pretty played out and boring, and we have darkin champions that possess a single person and are easily able to take control, so a darkin fighting a person he's already possessed for control wouldn't make much sense. In that way, I understand why they decided to make "Varus" two people instead of one.

But why not use the son?

The father defends the temple, runs back home, and finds his wife dead and his son dying. He can't imagine living without his son, so yada yada they both get possessed.

This would require the father being aged up a bit, probably to between his late 30's-40's, so that his son can be in his teens.

This would fix the issue of making the family non-canon, it would fix the issue of just retconning a character's sexuality, and it would maintain the interesting concept of a demon possessing the combination of two bodies while also taking the far more interesting father/son fights evil dynamic, than the done-to-death "Romantic love conquers all" thing.

It also would just make a lot more sense from the Darkin's perspective. Val and Kai are both pretty fit archers. Either of their bodies should be good enough for the Darkin. Why would he bother fusing the bodies and fighting two souls when he could just take one and have an equally fit body with only one soul? Having the two men be radically different ages, one too young and the other too old, would lend legitimacy to that decision.

I think a lot of people don't give the lore team enough credit. It's been a couple days for us, and I'm sure they've been mulling this over for weeks. I can't imagine they didn't come up with this idea of the father-son fusion, I just don't understand why they chose the current iteration instead.