Once you have the Oak's Letter you will need PkHex next, load up your SaveData.bin and click SAV > Misc Edits
Export your SAV and replace your current SaveData.bin and you should be good to go
This has been asked many times and people still don't search in the thread that Arceus files has been removed, you can say that so-called "Youtuber" is using a Pokemon Encounter cheat to make Arceus appear
There is no legit way to get Arceus yet
You can see the missing moves if you don't see them on your pokemon by talking to the move tutor at Pastoria City, exchange for 10 heart scales and will relearn moves for free onwards
It's already updated, Google search PkHex and it should be there
No tutorial needed actually, all you need is your save file and EdiZon, activate National Dex from PkHex
That I cannot confirm since I already completed the game, you can test it out if you want but I'd save my game before trying it out before breaking the game's code