Yup. Can only raise readiness with multiplayer! I just finished game 10 mins ago. So fu**ing sad right now. I picked the "destroy" option (red).
I was reading about it online and some people have said that you can still raise the galactic readiness in single player mode but the advantage of doing it in multiplayer is that it saves you time from having to do it in single player.
I'm not through yet. I started over because I accidentally told Miranda that things were still going on between us but I want to stay with Ashley. Yes, I'm that petty.
I love this game so much. It's really an incredible and detailed universe that you get to explore in these games. I honestly just spent about an hour going around the Normandy talking to everyone and looking at everything the first chance I got.
I'm really enjoying the "It's good to see you" moments with all your previous allies even going back to the first game.