Arches are a version of the "stir the pot" exercise that simply eliminates the bottom part of the circle made by your arms. This action is unnecessary and only makes the exercise more awkward.
When performing the walkout, roll the ball out in front of you as far as possible without allowing your lower back to extend, or as far as possible without feeling discomfort in your back.
When doing the pike part of the stability-ball pike + push-up combo, use your abs to raise your hips toward the sky while keeping your legs fairly straight. Raise your hips just barely above your shoulders. This will help maintain constant tension on your abs.