A terrible dinette table. Opinions on modification?

Have you thought about using a locking turntable bearing, or perhaps a flat-mount slide bearing? A slide bearing would require some sort of fabricated locking mechanism though.

A turntable bearing would allow you to rotate the table so the corners should be out of the way when you sit down. With a slide bearing the table could be just pushed toward the opposite side, the slid back after you are seated.

Wishing you good luck in overcoming this challenge.