My apologies ahead of time for the wall of text.
@tblst
"The house game was before Clara came to appreciate the other two as friends..."
I believe the house game was before the author decided to change her mind. There is another manga (by another author) where I believe the author changed his mind on a pairing (Yuusha ga Shinda! is the title of the work but I won't mention the pairing as it might be a spoiler). The author of the novel "Kono Subarashii Sekai ni Shukufuku o!" decided to change his pairings (or atleast make one official after some advice from another author). I'm too lazy to verify this but I've also read that author of Naruto decided to change his story after he had a son. My point? Authors change their minds (on pairings, storylines, etc) due to various reasons, and in my personal feefees this is the case with the Clara-Iruma pairing. It's not just the house game, if I recall correctly there were many panels where Alice and Clara were shown together (Alice helping Clara, teaching her things, etc; and it's been shown many times that Alice doesn't help other characters unless requested by Iruma himself). I would have to re-read the manga to come up with more concrete examples but I have no motivation to do so currently.
"...at the very least Clara does have a legitimate reason to like Iruma more than anyone else precisely because he was the first one to cherish her as an individual."
True, Iruma is Clara's first true friend. Although I sympathized with her situation (getting taken advantage of by fake friends), I still find her character to be annoying. To me Clara is like Tomohiro Nagatsuka from "Koe no Katachi" but more annoying. If Clara was to have a personality change/growth, I might like her better as a character but still would still support Iruma-Ameri pairing more.
"...it's completely realistic and reasonable given that they are demons who follow their desires and instincts..."
True, I hadn't considered that!
Although I think this was more of the author showing fanservice (Iruma-Clara ship, Lied-Elizabetta ship)/potential future pairings (atleast Lied-Elizabetta) than anything.