Can vision be corrected further/ made clearer by wearing glasses if it is already 20/20?

yes and no. If one already sees 20/20, there is little point in wearing a correction. However, theoretically, the vision in a young healthy eye can be as good as 20/15 or even 20/10 (very rare). So, if someone is extremely discerning, or has very high visual acuity requirements for their activities, it may rarely make sense to wear glasses or contacts when the vision is already excellent.
sometimes. The ultimate acuity depends upon the photoreceptor packing (like pixels on a video screen). The optimum corrective eyeglasses can match the packing and for those lucky enough to have tighter packing the acuity can be corrected to better than 20/20.