I don't feed dog food so can't help you there. I know that Castor & Pollux is one that LadyJane likes and feeds to many of her dogs and they do excellently. Don't beat yourself up about Science Diet. I disagree that it is a bad food and it is not a bad company - but it hasn't been the greatest for yorkies due to some of the ingredients and many like other foods better.
As for teeth brushing - Make it fun! We like to use the CET vanilla mint toothpaste by Virbac. My dogs readily accept the toothbrush and it has paid off. Barney is 10 years old and has teeth as white as a puppy and has never needed a professional dental! I also give lots of treats that help to clean the teeth like baby carrots, celery, and bits of apple.
Here is Teddy's video of how he likes to brush his teeth and you can see he is very good at it.