No. You need 3100 readiness which you can get with no multiplayer at all
Really? I've seen a few people that have finished it, done most of, if not all the side quests, and they still had to do multiplayer to get the best ending.
I may be mistaken though.
You don't like the franchise? I found the first 2 pretty good, but I just can't make myself like 3. I don't even know why.