Yes, a foal can die before birth.
Not without a very careful and costly ultrasound examination.
A mare has an insticnt and will not I repeat will not become pregnant before the foal is weaned but that is why a stallion that the foal is not sired will try to kill the foal so he can sire another foal of his own.
YES many horses have died giving birth so has the foal
To prevent the foal from becoming infected with Parascaris equorum you would have to clean the udders and teats before foaling.
=no you can not take a mare away from its foal straight after its birth=
A foal
A foal typically stands within 1-2 hours after birth, but it can vary depending on the individual foal and circumstances. It's important for the foal to stand and nurse soon after birth to receive vital colostrum from the mare.
The foal should start moving about the 4 or 5 month but it will be hard to see or feel at this time. At about 8 to 10 months you may see the foal move or kick the mare's side.
The soft, jelly like sole of a newborn foal's feet are called Golden Slippers, and are there to prevent the foal's sharp hooves from damaging his dam before or during birth.
To Foal~
waxing is when a few days before a mare is about to foal she will have a kind of waxy stuff coming out of her udder it will help let you tell when she is ready to foal this most likely will happen a few days before foaling
Usually one, occasionally two although twin pregnancies generally abort before birth or one foal is dead at birth or dies shortly after birth. Often both twins die. Successful twins are rare. Most breeders use techniques to prevent twin pregnancies.