the following reply are clearly base on Night's personal opinion
There are also new scenes if you choose new game after clearing a route. (this method gives significant to the skip button)
About the route locks, Key just want you to focus on your goal? which is helping Rin and figuring the secret of the world.
At least that's what I think.
By the way, my favorite route was Haruka and Kanata, the drama there are intense after all.
Indeed, her ending is kind of... how do I say this.
Anyway, my opinion is... wait, spoiler