To retain flexibility, do your stretching exercises between lifts. Take the day off. I count it as such, because your day of rest is designed with a purpose. It's to get you ready for your hard weekend workouts. Quality is the key to success, one reason I emphasize it in this Spring Training program.
Rest is important to maintain quality.
At the risk of giving my secrets away, no. Most important is to have a plan. Balancing hard and easy days with rest is part of my overall plan. In the meantime, simply have faith. And enjoy the workout.