Schneiderlin's was a monumentally stupid challenge to make, regardless of whether it was a yellow card or not. By unnecessarily sliding in like that you're giving the opposition player a chance to go down and the ref a decision to make. He was clearly angry with Aguero for milking it, which I believe he did, but to think that an opposition player won't take advantage of it when they are down by a man and a goal is very naive.
Same goes for Walker, I'm not convinced it was a second yellow but why look at the player multiple times before backing into him? It makes it look much more dodgy that Walker kept on glancing round to see where he was, because that's normally what a player will do prior to deliberately fouling someone. Sure, he could have just been trying to get his bearings, but he made it look worse than it was. Again, Holgate did him absolutely no favours with his over-the-top reaction, but I think that's to be expected nowadays in football.