1 - if every born baby is muslim by default, why they say azaan in the ears of newborns to make them muslims?? a ritual that is observed eachtime a child is born in a muslim family.
Humans like rituals/traditions. We invent things to mark something special (baptism, birthday, marriage, etc).
2 - why is that a baby has to be circumsized to make them muslim??
they say you are not a muslim if you are not circumsized.
No one is born circumcised, and it's even haram to alter/mutilate allah's creation. It's one of many contridictions.
3 - how many years a child remain a muslim after his birth? i mean a baby born in a non muslim family remains muslim until 12 or 15 or until the day he is baptized into another religion.
There's difference of opinion.
4 - obviously when he is baptized, he is not asked for his consent.
why muslims say a word on this "forced conversion"?
they mention being forced to be a muslim? Both answers are equally stupid.
5 - when you google muslim population stats of the world, it says 1.5 billion or somthing. do they include these " muslims by default" children?
if not; why?
It will depend on who's asking.