With that bracket spinning as you say I can see your pain. Can’t do anything about the indents but if the bracket is bent similar to a “V” then the ends wouldn’t grab like they need too. If they are ok I’d try screwing on two 1/4 inch nuts behind the spoiler nut used to retain spoiler. Leave them close to the end and tighten them so they’re tight against one another and use these as a way to hold bracket in place as you tighten the spoiler nut.

Hopefully there’s enough room to tighten with a wrench.