I might recommend colder instead of more cold. English comparative adjectives work much like German ones.

You can definitely use that in a song, if it helps with the flow use it. That wouldn’t be grammatically correct but song lyrics rarely are.

You can use it in a song. There are many songs or verses with incorrect grammar. On top of my mind- I can get no satisfaction. But often it is you and me instead of you and I and other way around etc. So don’t worry about it in a song