「Kyoko is a middle class 15 year old girl. She's very distant, cold, and unfriendly. She calls her older step-sister [given name]」
This would be pretty unusual. In Japan, younger siblings usually don't call their older siblings by name. It's generally oni-san/oni-chan/aniki when talking to an older brother, and one-san/one-chan (and in some rare cases aneki) when talking to an older sister.
A younger step-sister calling an older step-sister by her name is something that could theoretically happen, if the second marriage happened when the younger step-sister was no longer a little kid. Calling her by name instead of one-san would probably give the impression that the younger step-sister is adamant about not accepting her older step-sister as part of her family, though.
That said, calling the older step-sister just by her name (with no honorific) seems like it'd be unlikely in Japan. It's two steps below what would usually be polite (one-san → name + -san → name only), and it would go beyond simply being "cold and unfriendly" and come off like she's actively trying to start a fight with the older step-sister, which doesn't really fit with what someone with a "distant" personality would do.
「Mikan is a working class 9 year old girl. She's cheerful, friendly, and both intelligent and mature for her age. It seems like it's common for kids to call other people (even adults) by their given names, so I was thinking that she'd do that too.」
Kids calling adults by their given names, especially without any honorific, is pretty unusual in Japan. Generally, kids call male adults oni-san/oni-chan (for young adults), oji-san/oji-chan (for mature adults), or ojii-san/ojii-chan (for elderly adults). When talking to adult women, those become one-san/one-chan, oba-san/oba-chan, and obaa-san/obaa-chan.
Kids do often combine the adult's given name with these honorifics, though. For example, kids might call Taro "Taro-oji-san" or Hanako "Hanako-one-san."
「Yuzuki is a working class 17 year old girl. She's arrogant, vain, dramatic, and a bit of a bully. I was thinking that she could call people [family name]-chan as a way of being condescending.」
I'm not sure that calling someone by their family name plus -chan comes across as condescending. If anything, I think it'd sound flirtatious. In particular, a 17-year-old girl calling a guy by his family name plus -chan feels like something she'd be doing to try to show off how mature (since she's using family names) yet playful (since she's using the affectionate -chan) she is, or at least thinks she is. Speaking as a guy, if a girl I just met called me by my last name and -chan, my knee-jerk reaction would be that she's coming on to me, and in a fairly blunt, aggressive way.
My take on this might not be applicable to all people/situations, but it seems like I hear -kun being used far less often in actual spoken Japanese over the past few years. I realize it still pops up all the time in anime and manga, but I think that's primarily because it's a useful dialogue shorthand to quickly show a characters' personality/relationships.
Personally, the main situation in which I hear -kun being used is when adults are talking about boys. Things like teachers calling male students -kun, or parents talking to their own kids about the kids' male classmates/friends and using -kun. "Tanaka-kun, please hand these papers out to the rest of the class." "How was preschool today? Did you have fun playing with Hanako-chan, Taro-kun, and all your other friends?"
-kun occupies a strange spot in the Japanese language because it's an ostensibly polite form of address, but it's exclusively used for talking to/about people who are either below you or on the same rung of whatever hierarchy you're going by (age, school year, position in the company, etc.). So it's simultaneously respectful and not respectful, which makes it tricky to use.
Because of that, it can sometimes come off feeling sort of like a backhanded compliment. During my first job in Japan, I once called a male coworker who I was on friendly terms with (we'd hung out a few times outside of work) "Saito-kun," which resulted in everyone else in the office laughing and instantly giving me the nickname "bu-cho" ("section manager") because calling him -kun made it sound like I was his boss.