Actually you do need to play multiplayer, I downloaded the From Ashes DLC and did tons of side missions and my military awareness didn't move from 50%. I then proceeded to play through a second time with a rank 27 commander, got to the end again and it was still at 50%. I found all war assets, did all side missions and completed the game 2 times over and I still didn't get the ending where Shepard lives.
Here was my first ending: I didn't find all war assets, so I chose the "Control" option, when I did that Joker, Ashley and James survived on an unknown planet. I died, and the Reapers left earth but the mass relays were destroyed and all of those battle ships above earth were left stranded.
My second ending: Synthesis, a combination of organic and synthetic life into a new strain of DNA. The Reapers left "green", Joker, Edi and Garrus survived and the mass relays were destroyed, all of the battle ships above earth were still left stranded.
I then replayed the last 2 hours to get the final ending.
My third ending: Destroy. I destroyed all synthetic life, including the Reapers, Geth and Edi. The mass relays were destroyed again, the battle ships were stranded again and the Normandy crashed on an unknown planet, the door opened and then it cut to black. Credits rolled and I got the "Special ending" which was a deviantART picture which was edited in photoshop where an old man was telling his grandson/grand daughter about Shepard's tale. Never got the extra scene where Shepard can be seen breathing.
So I then proceeded to play multiplayer for about 3 hours. I won 10 matches, lost 3. My military awareness then jumped to 100%. I did the full 2 hour ending again and this time I got the special 5 seconds of video where Shepard breathes.
That was my experience with the game.