Then say that he's living in a -- to him -- strange arrangement of however many different families living in the one building and sharing the kitchen, and show what he feels about it (eg confusion because he can't work out who they are and what they're doing there). It still doesn't mean you have to name them and their relationships all at once, and you certainly don't have to look up videos of people eating breakfasts in order to decide what they're talking about.
You seem always to be putting the cart before the horse. Your plot should be moving forward and your characters should be doing things. Concentrate on what's important instead of wasting time and energy on elements that go nowhere and add little or nothing to the story.
Everything is strange to him, but this is picking an element that is strange to the reader as well. How would you handle explaining that a bunch of unrelated people are all in a HMO? Or would you ignore it and have a character outside of the MC's family standing in his kitchen like they own the place?
Maybe I am getting too concerned about details, but it's hard to tell which ones really matter and which ones will cause the whole thing to collapse like a Jenga tower. Like if I write an entire story based on a horse surviving falling off a cliff, (I'm pretty sure they don't survive falling more than a few feet,) the entire story might be unsalvageable except for the subplots. (Okay, I figured out an easy way to patch it just while composing this reply, let's call it an hour, but that doesn't invalidate my point.) You are right in the specific case about it doesn't matter what they talk about over breakfast, I needed more of a mood where later changing them from having a civilized conversation to talking over each other would have changed MC's reaction and possibly character development.
I did happen to find isitnormal.com which seems like a good place to ask about weird stuff that really doesn't get presented on the internet because it's so mundane.
I'd agree that unless and until these other characters actually do something that's important, it's better to leave them in the background.
One of the kids is going to blab on MC talking to the elf when he's not supposed to. Granny isn't actually anyone's grandma, but she gives a hint that the MC's master is expecting to be taken-care-of in his old age, and she's telling me that she actually needs to be there for something else later. They'll need to be introduced at some point, or could I just have them be in the room and seemingly come out of the background like a stage-ninja when they have something to add?
It really doesn't help you by referring to other media for this or any other problem. Film, TV and graphic novels can get away with things that novels can't eg having lots of people on the scene, because our eyes can quickly take in the differences -- sex, colour of skin and hair, body shape, clothing, voice -- which the written word takes longer to convey. You have to work within the constraints of the medium.
Having said that, I really don't think it's of much use to you in referring to novels either, when it comes to what other authors have done in a scene and therefore why you should be able to do it. Analysing the scene might help, to see how and why it works, but there are likely to be too many differences in when the piece was written, the author's style and ability, even the author's fame and following (established authors can get away with junk at times, because their work will be bought regardless) to make the comparison meaningful.
I do concede that there are limitations in each medium that have to be adjusted for, but is looking at movies completely useless? The details might be different, but are there things that absolutely can't be done when crossing mediums? (There are budget constraints, but are there differences between what can be done in a written real-world novel versus a movie?) Maybe a novel is better at avoiding "as you know" conversations. I started reading the e-book sample of Graveyard Book and it also has a ton of characters being introduced at once.
I have thought that just reading books wasn't the complete path to be able to write them, but why do people harp on it being such an important step when standing on the shoulders of giants won't give me the ability to pee further? Is there a literary equivalent to distinguishing between a fine painting and a drop-cloth that was declared valuable art for money-laundering purposes?