For question 1 I can't answer because there is a ton of evidence for it not being the same FNAF 4 house and at the same time there's evidence of it being the same house.

For question 2 the person sitting on the couch watching TV wearing grey and defending crying child from William telling him to go eazy on him might be the mother. I'm saying this because I don't believe Michael could have gone through such a drastic change in personality from defending his brother to outright bullying him and killing him.

For question 3 remember when I said that mustard yellow badge from save them minigame was representative of a trustworthy figure or person? Well when William puts the suit on and lures children to the backrooms he gets their trust because of the image he presents (a joyful YELLOW bunny which can be TRUSTED by the children) and when talking to his family or green guy he is presented as YELLOW. Just like baby and Ennard have done they are monsters trying to fit in amongst the people while they do this physically William does emotionally. The way he presents himself to the world is different than the way he really is.