Kallen's bond with Lelouch could have stayed based on loyalty, admiration etc. I would have been fine with that. When the staff developed Kallen's character romantically, I thought it was an understandable/logical way to develop her that way. They didn't need another love interest since there were already two possible candidates to handle that. I find their relationship to be truly beautiful to my eyes.
Lelouch did become her world as even noted even during turn 21. She needed an answer from Lelouch since she knew she couldn't move on otherwise.
Well, Lelouch and Kallen are quite similar to one another. Similar reasons for initial starting fighting, how important people in the past are and always putting others ahead of themselves. Not surprised they had such a strong bond without spending much time together.