Unanswered questions at the end of Durarara! (spoilers marked)

I always just assumed the dragon zombie guys were just hired by Izaya as body guards after he was stabbed, that's also when he hired the crazy knife bitch, the karate girl, etc etc. As for the karate chick she was the sister of the big karate teacher of izaya's sisters (idk maybe some connection?), for Aoba I thought his plan was to take over the dollars from the inside with whatever remained of the blue squares so he can fuck with his brother iizumi, I assume Aoba thought mikado was an idiot pushover and saw his opportunity. Thats what I thought, probably wrong but makes sense to me.

When in doubt, assume that the source material goes into more detail and background than the anime adaptation because they always do. Light novels (and to a lesser extent, manga) will always be more detailed than their anime adaptations. Written mediums allow for a lot more detail than visual mediums, especially mediums like television which have built-in time restraints (episode length + count) and storytelling limits that simple can't convey the same amount of information that a written story can.

So - yes, the Durarara!! light novels are much more detailed than the anime. And if an anime adaptation of the sequel series Durarara!! SH is ever made, then I guarantee you the SH novels will also be more detailed than the anime. Same with the Izaya spin-off series.

Also, a lot of your questions can readily be answered by consulting the Durarara!! Wiki.

Q1: "Karate girl," aka Mikage Sharaku, attended the same high school as Izaya and developed a crush on him during that time. As you can imagine, Izaya didn't reciprocate and used her like he tends to use most people. This culminated in Mikage shouldering the blame for a scandal she didn't cause, but sorry - we don't actually know what the scandal actually was. Just that it caused her to drop out of school.

Q2: The Dragon Zombies are a huge Ikebukuro biker group led by Li-Pei Ei (who is probably related to the Ei family from Etsusa Bridge, fun fact). Normally they spend their time drag racing around the city, but their relevance to the story begins when Izaya hires them to act as his bodyguards in the aftermath of 'Yodogiri' stabbing him.

That's why several Dragon Zombie members arrive with Izumii, Haruna, and Mikage during that scene in which Izaya was tied to a chair and briefly waterboarded. They're there to both ensure Izaya's safety and help deal with the members of Heaven's Slave and Amphisbaena. Later, a few Dragon Zombies help Mikage and Kine in their rescue of Izaya from Saika-Slon.

Q3: Bald guy's name (alias, not real name) is Kine. He used to be with the Awakusu (you know, the yakuza group that Shiki and Akabayashi are in). I believe he first met Izaya when an in-progress fight between Izaya and Shizuo ended up stopping his car in its tracks. Somehow he manages to diffuse the fight.

The details aren't clear on this, but Kine eventually warms up to/takes a shine to Izaya and introduces Izaya to the Awakusu. (I think he also had a hand in Izaya's info broker career kicking off?). At some point he leaves the Awakusu to become a freelance detective. I'm pretty sure he's one of the only members of Izaya's alliance who doesn't actually hate or have it out for Izaya.

Q4: The reason Aoba asks Mikado to become the leader of Blue Squares is because he plans on using him like he used his brother Izumii way back when. His aim is to restore the Blue Squares by having the Dollars become part of the Blue Squares...but when Mikado later on stabs Aoba with the pen, he realizes that the Dollars ended up absorbing the Blue Squares instead. That's when he starts helping Mikado cleanse the Dollars, though it's not like he's not scheming.

Q5: I... think you must be referring to Shu Aozaki, the Awakusu member who wears a red shirt in the anime and is rivals with Akabayashi. In the anime, he knows that Mikado is the leader of the Dollars and plans to eliminate him so he can install one of his subordinates as the new leader. As a result, he would be able to turn the Dollars into a revenue source for the Awakusu. If you'll recall, he gave Mikado a gun.

But Akabayashi, clever Akabayashi, calls him out on his schemes and asks Aozaki to let him handle it instead. That's when Aozaki receives news that the shots fired at the Awakusu boss' home were from a much smaller caliber gun compared to the sort he deals in. If I remember this right, he'd been thinking that Mikado had fired his gun at the house - but after hearing the news, there's no longer cause for concern.