actually cooling is not the effect they are probably after, its heat,, carbon brakes are crap till heat gets in them its usually duct for dymanics and retention of heat
as soon as its rain/wet they swop back to steel disc/calipers setup ie if race started wet no one would probably be out there on carbon setup they cant retain the heat and or braking effect.
They are mainly an aerodynamic feature, helping to work with the front wheel wake. They are very complicated and to get them to work well with the rest of the aerodynamic package takes a lot of work, for little gain, which is why a lot of teams have not gone for them yet, or have tried them but then gone back to not using them.
There is a balance to be made between the very slight advantage they give over a race distance, and the chance (as with Ferrari last year) that you loose more time in the pits when trying to get them off, when changing the wheels.
As to the other point, F1 all use carbon brakes, none of them use steel brake discs under any circumstances. The weather makes no differenvce to how the cars brakes work.
F1 teams are no longer allowed to change any of the brakes after qualifying and before the race so all qualifying is done on the race setup now.
F1 have not used steel brakes for at least fifteen years IIRC.
The new wheel covers are certainly not for retaining heat, in fact one of the most annoying things about them from a mechanics perspective, is that they retain the brake dust which then goes everywhere when you have to change a wheel, much more so than withouit a wheelcover.