Q: Reset highest peak value in the box above the audio meter?
A: Just click the box.
Q: Add a new mixer strip?
A: You can not do that directly in a mixer strips window. You either have to create a new instrument in the modular mixer, or add a new instrument to a track in the editor.
Q: Switch between a narrow and a wide strip?
A: Left-click the name of the mixer strip.
Q: Delete a mixer strip?
A: Right-click the name of the mixer strip, and select "Delete". Or shift+right click the name.