Regarding refs on Man Utd-Man City match, it is clear that FA and refs "adjust" their views based on what they need in particular season.
As someone already mentioned, FA probably wants Man Utd back, because they are the biggest English club who brings tons of money.
And they will also push Arsenal back, to a title. From the same reasons, huge English clubs, it is better to inject their fans some fresh blood.
In Italy, the same happened with AC Milan in the last 2-3 seasons. Last season when they finally won a title, they had a pen more or les in every match for every single contact.
Why?
Because Italian FA needs big AC Milan and they need Milan back.
It will bring back more prestige, more money, more buzz, more viewers longterm to Seria A.
In Spain, refs are helping to RM, but not for the same reason as in England and Italy.
In Spain it is because politics (Catalonia is not Spain).
And then you guys wondered about my rants during a WC, helping to some big teams, and how someone out of 3-4 big/brand teams needed to win, helped by ref's pens whenever possible.
Money money money money money money money money money.
Football is a sport only up to a certain level.
After that it is rigged by interests, sponsors, money and future growth on the back of star players/poster boys.