you can find mice in caves you visit with Agnes or the caves under the Tavern. the Shift button is the dash button which you can also turn on in the options.
If you are having speed issues might be due to your frame rate or something
You first have to find the child of Ms Eve. Check the quest board in the village.
The child can be found in a cave in the inner forest, near the watermill. After you found it you have to ask Father Fox for an antidote. He will tell you to visit the Oracle in the swamp.
After that you can go to the swamp.