So in the end, it looks like you need the multiplayer in order to get the best ending - and at least one of the achievements. I'm near the end of the game, exhausted most of the content, and have a readiness of 7000. Cut in half, that's 3500. It's still enough to fill out the bar, but given that the achievement related to that doesn't pop until it's higher, there's an indicating that you still need more readiness points that simply aren't available.
You know, it's not that I'm likely to replay the game later on, but I wonder what happens when either no one is playing the game online or the servers are shut down.
On another note, I'm disappointed that they didn't include a full character creator with the ability to pick out the choices from the first two games - especially since the only way to get some content is to have a character who made very specific choices in ME1 and ME2. At the moment, short of replaying the three games, you're stuck watching videos on youtube if you want to see some of the alternate scenes.