Since she is so young and a maiden, it's recommended that someone be there when she foals. In the slightest instant, something could go wrong and you could lose both baby and momma. They usually foal at the worst time possible, when your sleep deprived and need to pee. You come back, there's baby.
It sounds like she's close, but so many mares have so many different signs of foaling, she could be a few days away, or she could be a few months away still. Without knowing when she was bred, there is no way to know for sure. Even when you know specific dates of when the mare is bred...well...they throw you for a loop and go early or go later.
What does her vulva look like? Inside and out? Inside, will typically turn a dark red. The outside will elongate and become very wrinkly looking. Her tail head and croup will relax and become very "jello-y"
Since the vet said the baby has moved into the birth canal, I'd be ready. Have a foaling kit on hand.
I would call up the previous owner and tell them straight up..."Look, this filly is in foal. It'd be nice to know what dates she was bred. I've had 2 vets confirm there's a baby in there so there's no trying to deny she was bred. I'm still keeping her but if you could tell me the dates she was bred, that'd be extremely helpful for me to have a better understand of when she should be having this baby." And I'd have a no-nonsense tone.
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."