You would be surprised what a "fake breeder" would do. Taking pictures and doing a pedigree would be simple, I don't why you would think this equals a good honest breeder, even puppymills do this. Maybe your breeder intentionally lied to you or maybe she was duped like you. What makes you think she knows anymore than you do whether she has a purebred yorkie or not? If she was told she had a purebred yorkie and had some papers for it, she probably didn't question it and just assumed she did, so she bred them.
My honest opinion is that she is a chihuahua or a mini pinscher with some yorkie mixed in. Her newest pictures confirm that as she looks even more chihuahua/min pin now than she did when you first got her.
I don't think her hair will ever be long or fluffy. Her coat texture isn't even yorkie, it is chi which typically stays short unless it is a long coated chi.
I found these two pictures for a comparison for you. They both look a lot like your girl and I would say this is how she will look as an adult
This first one is what many chorkies (yorkie/chihuahua mix) look like...
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This picture is a chihuahua/min pin mix
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I know it may be hard to accept, but I don't think she will ever look like a yorkie any more so than she does now. Which is fine because she is absolutely adorable the way she is and you can tell how loved and happy she is!! She truly has such a sweet face and I can see why your family fell in love with her. Just accept her for what she is and love and spoil her.