Tell us about yourself!
Gluing and Fastening PVC
This is impressive work.
For strength, it would be better to cut both horizontal pieces shorter and the vertical pieces longer and put the horizontal pieces between the verticals and put the dowels in horizontally. Also smaller diameter dowels would lessen the chance of breakthrough if the dowel holes weren't drilled exactly parallel to the face of the boards.
If the shelf was made about 3/4 inch longer, so the distance between the keyhole hangers was 32 inches, then the shelf could be hung from screws in two wall studs (wall studs are typically 16 inches apart). This would be a sturdier hang - and would solve the concern about the cat!