1. usually, 2 stops down from wide open is the optimal aperture for enlarging lenses. If your enlarger is properly aligned and the negative stays flat, you should not see any real difference in the results whether you use f8 or f16. The f-stop range is useful for managing the length of time of your exposure.
3. Anti-newton glass for the top glass in a glass negative carrier. It helps reduce the likelihood of newton rings appearing in the print. Not a pretty sight. If you use glassless neg carriers, not an issue, only flat negatives become an issue.