Mario Kart World - Mirror Mode Unlock (Minimum Requirements)
27I show at (2:00) that you only need 10 P-Switches, 10 ? Panels, and 10 Peach Medallions from Free Roam to unlock the Mirror Mode. With those, and having beaten every Rally (placement and ranking doesn't matter), as soon as I unlocked the Special Cup, I beat it and unlocked Mirror Mode, so this seems to be the unlock method for them.
One thing that I haven't tested is seeing if beating 50cc is enough to unlock it. However, I did test and made sure that just having 10 Peach Medallions (with no P-Switches and ? Panels) wasn't enough to unlock it.
If you wish to see me going through every Knockout Tour & Grand Prix on this save file, you can take a look over here: • Mario Kart World New Save Unlock Mirror Mode
Unlock Special Cup: 0:00
Free Roam, Knockout Tour, and Grand Prix Completion status: 1:58
150cc Special Cup: 2:51
Unlock Mirror Mode: 20:45