That sounds very scary, I'm sorry that happened.
I have a 2.5yo so I REALLY understand this age and power struggles over toothbrushing. I don't think forcing this is the way to go. Sometimes it takes me 15-20 minutes to get my 2.5yo to cooperate, but she always does eventually.
In the morning I have her brush her own teeth while still strapped to her high chair and I sing songs and dance for her as long as she's brushing. If she stops, I stop.
At night, she gets an assisted brushing, in her high chair and she doesn't like that part. But, my husband and I work together to distract her to the point of hardly noticing. I have a stach of toys that the ONLY time she can play with them is while she is cooperating with toothbrushing. Her favorite is a necklace. The idea is to keep both her hands occupied and her mind on something enjoyable while we do a really good job brushing for her.
The more you force, the more she will be scared and resist. You could be creating a lifelong hate of toothbrushing with this method your dentist recommended and that will do more harm than good.
You might start small and give the child TOTAL control back over her own mouth. Encourage her to brush her own teeth and when she makes ANY attempt, reward it. If you wouldn't mind, I'd like the name of the dentist that gave you that bad advice, so I don't take my children there.
We currently go to Dr. Titensors and they've never recommended anything like what you described. Just positive and encouraging the child to do as much for themselves and us to do one good brushing a day.