She doesn't have powers that is why his heart shrinks. She didn't meant to do that but becuase she could not properly read the spells thats what happened
No. Not everyone can read the book but anyone who understands the words can control it.
The reason why the spell went out of control was because Nessa did not know what she was reading or how to properly read it.
But anyone can control if it they know how to read it (Madame Morrible said for instance it took her several years but she cast a spell or two from it).