Welcome and congrats on yuur adoption

it can take some time. Allowing her to get away and relax when she needs to is good for her. One thing you can do is put a lot of toys down on the floor and even sit on the floor while studying or just during off time and see if she will come out to play. Tossing treats gently to her everytime she makes a positive move towards coming out, going potty outside, playing etc will help as well.
Btw good catch on the food change, my dogs had a hard time with blue (not every dog does) so if she does have a hard time with it I suggest checking out taste of the wild.
As for the teeth she may need a dental to get the tartar off so you can keep them clean with brushing.
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