[MazdaGT_E]

eventually loosen the 3M tape and the spoiler will fly off if that's the risk I'm willing to take.
The max speed I drive is around 90 MPH so does downforce really matter?
I know 3M tape is enough for ducktail spoilers but I don't know about the one I bought.

just a little fun Aero fact, the dodge viper ACR rear wing @80mph generates over 1000lbs of down force. I know your wing and the ACR wing are drastically different but still something to think about with highway speeds and Aero accessories and the forces they generate.

Put your hand out the window like a plane wing @ 90mph and see if you think thats enough force to rip a wing off thats taped on. Start with your hand at a downward slope then turn your hand until its level and watch how fast the wind rips your arm up.
I'm willing to bet that a wing just taped on wouldn't last one trip at 90mph.

I would NEVER trust just 3m Tape to hold down a wing, Maybe a little wing extension but not a full wing like the one you bought. plus its easy to steal or vandalize when its just held on by tape.

Just my opinion.

Why dont you want to drill the holes to install the wing?

 

[MazdaGT_E]

Ok, noted.
Is there a specific size of rivet nut that I should use?

Sorry if I'm asking stupid questions, I've never done this before 😓

I would go with M6 Riv-nuts and bolts, those are a pretty good bolt and size, if you need to add them to the wing. Try threading a bolt in thats the right size ad see what happens, if it threads and feels tight then you're probably good. if not add riv-nuts.
drilling into your car can be unnerving but there are plenty of youtube videos to help guide you. as far as rust goes you can tape off the areas around where you drill and spray a little bit of rust preventing primer on it. and as mentioned above use the tape to seal off the holes after.