Im going to edit my comment and answer the best way I can
1. Scott hasnt officially stated , which sparked all this , and the books suggest both ,pretty much Scott is setting up this huge question with one of the answers being a huge red herring , and hes just making it more confusing while it goes on, theirs also a name for each side , for those that believe mike is foxy bro,(mikebro) pretty much they believe he's on a huge redemption quest or whatever , I don't understand them so I can't speak for them , and then theirs those who believe mike is cc(mike victim) which means he's just a regular kid who or bullied , had a terrible family ,was trying to survive , and was there to free those who his father caused pain too, im personally a mikevictimer so to speak
2. Their are hints here and there , if your a Mike victimer the case is open and shut , his names Mike , BUT , If your a Mike broer, the real mystery sets in , scott has never once slipped uo about the name , he never has said it so far , although theirs a theory connecting a name to him its not confirmed , Chris is a fanmade name ,the best way to phrase him is bv, cc(bite victim and cc ) or if you believe the theory I mentioned Evan (which you don't have to if you don't want to )
3. Depends on what you mean , if you call the older brother of fnaf 4 Terrence then yes he "exists"
If not and your talking about them having a secret other sibling other then Elizabeth ,cc , and foxy bro not mentioned in games no,
Theirs only three Afton siblings , foxy bro, cc, and Elizabeth , the name terrence actually came from a excellently drawn and tension filled video from a long time ago
4.
They add aftons mostly because of the same reasons people make fan fiction and other things ,
1. To make money (jk, partially )
2. To add things they love together , they want to add things they like and make some thing new , and help others who also want to see something new , for example izuku Afton (im part of the mha community so I do understand them and know why they like it) is such a good concept because they like the idea of deku turning evil and villain deku and things among that nature ,villain deku is actually one of the most loved concepts there, not saying I hate it either because seeing how your favorite character would react in a different situation is nice ,and their superheroes so seeing how they would react here , wondering to help the villains who have been through so much, or to arrest/kill them for their crimes
5.Don't know who Beatrice is , who's it referencing , or whatever , but just know she isn't canon and is fanmade
6. ...
Ill tell you his excuse and you can decide for yourself , he accidentally killed both of his kids , somehow lost his wife ,and before killing /turning bad got spring locked , it changed him , and some other factors came in mostly because he went insane trying to find a way to immorality , had to deal with Henry having a child who was alive , and healthy , and seeing him be a father while losing 2/3 of his kids , and foxy bro possibly went to jail , and even if he didn't the moment he killed cc he probaly wasn't gonna win son of the year award , and he grew jealous of Henry , and theirs other reasons as well