#1 What exactly constitutes a penalty?

When Nicolas Otamendi made contact with Ander Herrera's foot inside the City box, three things were mighty clear
a) Otamendi got none of the ball and all of the Spaniard
b) Herrera was going absolutely nowhere with the ball
c) Herrera was quite obviously looking for it.
Now, as per the laws of the game when a) has occurred, does the referee even need to consider b) and c)? Herrera's progress was obviously impeded by Otamendi's errant foot, but does the fact that he may or may not have done something with the ball were it not for Otamendi's challenge have anything at all to do with it?
Which is why Jose Mourinho was furious at the post-match interview, commending Michael Oliver for a good game (he did have a decent one, allowing the game to flow where many others would have made it a stop-start affair) but saying Oliver spoilt it by this one, game-changing decision.
Pep Guardiola, meanwhile, swatted the question aside - as any winning manager would.
Whether City got the rub of the green or not, the significance of such slices of luck, such decisions going in your favour (each team has this, United themselves were lucky not to concede a penalty at Arsenal... and so and so forth) on the overall title race simply can't be underestimated.