Some people in CP fics use switches and rulers as disicpline, but that was never the norm in my house, so it goes back to my childhood, I guess.  Sirius knows nothing about how Dudley treated Harry, Harry never discussed his home life with Sirius at all.  In the letter, he mentions his abuse by Vernon, because that was the worst of it.  Dudley was following his father, and while he was mean and nasty, and a bully, his sort of meanness is the kind that can be corrected by a good example by a male role model.  All Sirius sees when Petunia and Dudley get there is a scared spoiled kid, one that needs major guidance, not an evil irredemable brat. 

People eat chicken pot pies in Britain too, and most wizards tend to eat normal food, in the books JK has them eating ice cream and chicken and roast beef, I've never even heard of a wizard main dish.  Remember, though they live in a separate world, the wizards also live in our world too a little, so why wouldn't they eat regular food? They might not prepare it the same, but they still eat it.

The keeping Harry's identity a secret issue will be addressed, but not right then.  Severus has enough on his plate with the upcoming trial and harry knows it, he's not being cruel.  Yes, Severus is loving here, he has no reaosn not to be, he isn't Harry's enemy he's his father and Draco's godfather, two things which substantially alter a persons's feelings, so he doesn't feel resentment towards Harry the way he did in the books. 

It took Draco and Harry two months to finally start getting to this point, I've mentioned it before.  In the beginning, they drove Severus nuts with their bickering until he put a stop to it by making them share a room and do lots of chores together. When you're with someone 24/7 and have only them to interact with and you're being told by your father to get along with that person, it's pretty hard to keep animosity going, since you start to see them as a person and not your rival.  Draco & Harry have been through life and death together in the Fae realm, something like that is bound to change a relationship, hopefully for the better.  Chapter after chapter of bickering is very annoying to write, I couldn't stand it in OOTP.