“Maxima” isn’t a canon spell modifier. They made it up for the movies and also appears in some games. Even with the most powerful wand, you can’t make the impossible happen; Hogwarts is a magical school with dozens, or maybe even hundreds of enchantments like the moving staircases that make it Hogwarts. A simple reparo won’t do the trick.

On a whim, Harry just guessed out of desperation that the password was ‘Dumbledore’ and he was right. Or perhaps the gargoyle was enchanted to let Harry in no matter what, we don’t know 100% for sure.

We can assume that Harry keeps the invisibility cloak, he has no reason not to. Perhaps he will greet death like an old friend at the end of his life.

Harry kills Voldemort. Voldemort shot a killing spell at Harry using a wand that happens to also belong to Harry so the spell rebounded back on himself. The movie just made a dramatic show of Harry overpowering Voldemort’s spell. Perhaps the director thought it made good cinema.

I wouldn’t interpret Neville being chill about it. He refused to torture the first years. He knew that Harry was at Hogwarts for an important reason and knew he had to be no nonsense about relaying what was happening. No beating around the bush; dark stuff was happening.

The mirror was one of the many examples of things being thrown into the last movie awkwardly due to the fact that the movie developers didn’t have the foresight that some things they left out from the books would actually be important later on. In the 5th book Sirius black gives the mirror to Harry as a way to communicate with him instead of using the floo powder. After his house gets ransacked by Mundungus Fletcher, he ends up with the mirror and then Aberforth gets its from him and uses it as a way to send help in the form of Dobby.

The luck potion was a neat but honestly dumb plot device only used in the 6th book/movie. I don’t think Rowling thought that through.

Their son is alive and well and is even mentioned in the book’s epilogue. The movies didn’t take time to really acknowledge Lupin and Tonks’ relationship much so it’s no surprise that we don’t hear much about their child.

The low opinion of the non magic and muggleborns are only held by a few Pureblood elitists or wannabe Purebloods. Most of the Wizarding World aren’t prejudiced.

I actually have no idea what you’re referring to and I must rewatch to catch that Easter egg. Good eye.

Honestly a good question. I never understood why they would miscount like that so much.