Actually I wouldn't let her foal out with other horses in the same pasture.
Especially if she's new to the already established herd. If there are geldings in the same pasture, some geldings have a dendency to possibly kill the foal. If out with other mares, some mares will swipe the foal from the mother and it will starve if not caught in time. I've had a gelding try and kill one of my foals before

, the foal was probably about 2 months old at the time. It wasn't long before that gelding found himself a new home. I've had other mares try to swipe other mare's foals before too.
If possible its best to have a separate pasture for your pregnant mare where she doesn't have to worry about other horses.
Might even be better if she wasn't turned out with the rest until a little while after she foals. Establishing status in a horse herd can get nasty. You wouldn't want to risk her getting kicked and losing the foal.
The vet can palpate or ultrasound the mare to see if she is in foal and can likely give you an approximate birthdate.
Weaning, all I ever do is just take the foal and put it in a separate SAFE pen away from the mare, where it can't see the mare. Or better yet, if you can sell the foal right off the mare, then you don't have to worry about the foal getting into fencing ect trying to get back to mom.
If the mare is in foal, if she's been used to being ridden then you can likely continue light riding up to a month before foaling. If she's not been ridden much then I wouldn't bother starting her back up now depending on when she's due. I wouldn't worry about her miscarrying, not likely going to happen being ridden lightly, but don't ride her hard everyday or put her in any intensive training.
If the mare and foal has good pasture or hay, I wouldn't bother giving the foal grain. Giving grain to foals can cause quite a few problems with development if not done properly.
If the mare is skinny, it wouldn't hurt to give her a small amount of crushed/rolled oats with some corn oil to slowly bring her weight up. Mineral and salt blocks should be free choice for her. I'm not a big fan of beet pulp I personally won't feed it to any of my horses.