Later, as the townsfolk celebrate their liberation, Steve hears his buddy singing in the local café and notes that it’s the first time in years – when, the next day, the sniper apologizes for being no use to the mission, Diana smiles and says “but who will sing for us?”

I like that.

Given how Steve is portrayed in the rest of the movie, I don't think the scene is trying to say "Steve Trevor is too footloose to settle down," I think it's saying "Steve fights to protect those who still have a chance to go back to civilian life, but he personally has given up any hope of that.

Interesting. My first thought is to wonder if this ties in with his ultimate fate in the film. My second is that your description of this scene chimes with something in a much older movie, but since I can't remember the line right now, I'll have to get back to you.