"You're the web in front of a favorite lie"
The key here is that it's "A favorite lie", not one specific to the Archer or to the girl he's addressing - the lie is happiness. In this case, it's happiness with this girl - he is getting caught in her web as he pursues this "lie".
"Stuck a pin in your backbone, spoke it down from there"
To stick a pin in something is to stop it. He's coming to the realization that this relationship is escalating on the basis of something not real - of this "favorite lie". The Archers stop this, and he talks her down - he's breaking up with her.
"All I ever wanted was to be your spine."
He just wants to support her.
"You're the web in front of a favorite lie" The key here is that it's "A favorite lie", not one specific to the Archer or to the girl he's addressing - the lie is happiness. In this case, it's happiness with this girl - he is getting caught in her web as he pursues this "lie". "Stuck a pin in your backbone, spoke it down from there" To stick a pin in something is to stop it. He's coming to the realization that this relationship is escalating on the basis of something not real - of this "favorite lie". The Archers stop this, and he talks her down - he's breaking up with her. "All I ever wanted was to be your spine." He just wants to support her.