1) Yes, you can stay with any one exercise as long as you want. You would not have to go on to any other exercise.
2) There is no harm in practicing in front of the mirror. It can help you with your form.
3) Walk any time.
4) 20-30 minutes is a sufficient wait time. It is best, too, to wait 20 minutes after exercise before showering. Since that isn't always possible, do the best you can. One day last week I simply ran out of morning time, and took a shower right away. Just make sure the bathroom and the water is not cold.
Enjoy.
