1- Alternates replaces humans with other Alternates (aka turns them to Alternates/clones them as Alternate) and after replacing them, replaced person dies.

2-Adam was replaced while he is 4 year old, while still he is a baby. He even forgets The Intruder so its no surpise that he don't know that he was replaced.

3-No, they also exist in other counties and maybe other countries.

4-Unknown, maybe He just doing it for fun or maybe other Angels don't know about whole situation and when they learn, they can stop Satan and his plans.

5-He may be taked it from his father's room and after that he locked himself to his room.