I thought the Maguire incident was a red card, even if it's off the pitch it doesn't matter - his leg shouldn't be making that movement unless it's intentional. I actually think their first ruled out goal was the correct decision.
Yes, Fred is pushing a moving Azpilicueta but he maintains balance and when he shoves a completely stationary Williams, he's not falling forwards but stood upright on both feet. In shoving his opponent, Azpilicueta is gaining an advantage as he pushes Williams under the ball.