Man, that scrap panel test only proves what you can SEE and FEEL, not what holds up after five winters of sun and road salt. A finer scratch gives the epoxy more tooth and more surface area to grab, and that shows up years later when a 320 job starts lifting at the edges and a 600 job doesn't. Your mentor proved it doesn't matter for a Monday morning, not that it doesn't matter at all, so I'd keep going to 600 on anything that lives outside.

Sounds like a classic case of people wanting to believe so bad they'll accept anything. Two phones and a time stamp prove the clips match, not that the light was some craft doing impossible moves. Phone cameras smear and warp lights all the time, especially at night, so a blob that "shot straight up" could just be a shaky hand or a lens flare.