Funny observations about Harry Potter´s books (Lo que no cuadra en Harry Potter)

So I don’t know if it is just me or what but I can’t seem to stop listening to the Harry potter audiobooks, I love the voice of Jim Dale and I love the story, of course!. I listen while I’m knitting, or while I’m doing something boring like working in the lab, or even to fall asleep on those nights that I can’t.

I have listen to them so many times that I have observed a few things that doesn't fit or that are left without explicit answers, so I begun to write them down and this is the list of what I got so far. Please feel free to reply (good or bad)

•  In book 1 Hagrid tells harry that James and Lilly were headboy and girl but in the fifth book Sirius saids that James was never a prefect
• In the seventh book, ron can say the name and address of his brothers house, shell cottage, but the place is supposed to be under the fidelius charm and the secret keeper is not Ron
• Again in the seventh book, dean produces a woolen hat when they put dobby in the grave but he has no wand
• In book 5, Tonks uses a spell to pack harry’s things and cleans hedwig’s cage, then uses some other spell to move the trunk and Moody puts a disillusionment  charm on harry, all this magic inside number 4 privet drive and the ministry said nothing?
• In book 2, Percy takes 5 points from Gryffindor, but in book 5 they say that prefects cannot take points from houses and Umbridge gives that power to the inquisitorial squad.
• In book 2, Malfoy says that the chamber of secrets was opened 50 years ago, before his father time. Was Malfoy in school with voldemort or with Snape?
• As a curiosity, the sixth book makes a lot of references to the second book
• In book 7 hermione says that she put a memory charm on her parents, actually she said that she modified her parents memories, and then when  they are in the cafe she says that she has never done a memory charm
• In book 7, the letters from hogwarts apparently don't reach them, the book does not say
• In book 1, Voldemort says to harry that his dad put up a great fight, but after we know that he did not even had a wand with him and that he did not fight
• Book 3, how did Fred and George "improve" Percy's badge without magic? Because they were still underage
• In book 7, when the guard arrives at number 4 privet drive they take away the disillusionment  charm that they have, why doesn't that counts as magic?
• In book 7, harry drinks "enough" polyjuice potion to last several hours in the wedding, but we have learned, or at least Dumbledore says so in book 4, that the polyjuice potion must be drank every hour.
• In book 7 there is the story about Kreacher going to the cave with Voldemort, but in book 5 Kreacher helped Voldemort  make the plan to lure Harry to the ministry, I know that he probably spoke was with Bellatrix and her sister but Voldemort was in on the plan, how did he not realized that this was the same house elf?
• In book 7, at the end, how come there was no portrait of Snape on the headmaster's office
• In book 7, isn't it a bit weird that the elder wand knew about the other one? They are not programed or connected; I guess that this is magic!
• So when you take the polyjuice potion your voice changes or not?, because in book 7, when they go into the ministry, they say that they are speaking in the persons voices, but in book 2 Harry and Ron must fake the voices of Goyle and Crab. Particularly in the audiobook there is this confusion because when the fake Harrys take the potion their voices don't change, but when they go into the ministry a few chapters later the voices do change
• In book 6, does it mean that everybody but Harry and Ron got an outstanding in potions? Even the Slytherin group? poor guys! J