Because they are in the legality box of the brakes. The winglet however abuses a clarification the FIA made allowing mirrors on the halo.
Definitely not, First of all, they haven't ran such a thing before. This alone would imply that they are very much capable of making structurally sound mirror mounts, making such a thing unnecessary.In a pinch Ferrari mightcould argue that the fins are an aero stabilizer for the mirror, an air-anchor, an aero-stay, if you will.
Second of all, aerodynamic load changes with speed, which would thus mean it's rigidity increases with speed.