Her face was definitely not thrilled as if this was the best moment of her life. It seemed mostly shocked and … well anything but pure joy is going to read as not enthused, but I feel like she was processing more than anything. I don't condone teenage engagements in real life but as a TV viewer it's not the craziest…

I assume she'll say "yes" but they won't get married for a while…years hopefully. It's just a way to further "tether" them to each other.

They definitely did it. I have it in my itunes. Puck and Will sang it at the end of Season 1.