How can one determine whether phrases in songs such as "I think of no one but you," "You are the oxygen for my life," or "You are my moon/my life" reach the level of shirk al-maḥabbah (associating partners with God in love)? And is someone who repeats them unintentionally considered a mushrik (one who commits shirk)?

Such phrases do not imply shirk (associating partners with Allah) in love. Shirk in love is the love of servitude that entails humility, submission, glorification, and complete obedience. It is advised for those suffering from obsessive-compulsive disorder to refrain from thinking about matters of disbelief, and it suffices for them to avoid prohibited songs.