One way to interpret Raku's birthday gift to Chitoge is that the doll is supposed to symbolize her. The doll has gorilla-like characteristics, but also a cute, feminine side to it.
Re-reading the story recently, I realized that the reason Raku is so incapable of noticing when people are in love with him is because before he met Chitoge, his yakuza family was so overprotective that they would threaten anyone who even came near him. Because he's been so sheltered by his family, he's simply incapable of noticing that someone is interested in him.
Raku's feelings for Kosaki can be explained in light of the other events of the story. Yes, they knew each other as children, and yes, she's the girl that most of the guys in the school are crushing on, but most importantly, she represents normalcy to him. His justified anxiety over the situations his dad and the Yakuza have created have resulted in him desiring nothing more than to marry Kosaki, change his surname to Onodera, become a civil servant, and never hear the following words again: Yakuza, Beehive Group, Mafia, SWAT team, assassin, and sniper.
The storybook is very important to the plot because of its similarities to and inspiration of the plot, but what may turn out to be even more important are the subtle differences between the storybook and the plot. In the storybook, there are only four girls, including the princess, but in Nisekoi's plot there are six. Also, the storybook only has one key, whereas the plot has three. These could be instrumental in Raku's rewriting of the ending.
Why would Ruri have Raku pretend to be her boyfriend, rather than Shuu? Because Ruri's great-grandfather is way too pushy, and Shuu is a pervert. Put the two together, and it would be a disaster waiting to happen.